diff --git a/cli.py b/cli.py
index fb44790832..febe327899 100755
--- a/cli.py
+++ b/cli.py
@@ -214,6 +214,7 @@ def load_cli_config() -> Dict[str, Any]:
             "streaming": False,
             "show_cost": False,
             "skin": "default",
+            "theme_mode": "auto",
         },
         "clarify": {
             "timeout": 120,  # Seconds to wait for a clarify answer before auto-proceeding
diff --git a/hermes_cli/colors.py b/hermes_cli/colors.py
index d30f99c62d..415db15911 100644
--- a/hermes_cli/colors.py
+++ b/hermes_cli/colors.py
@@ -1,5 +1,6 @@
 """Shared ANSI color utilities for Hermes CLI modules."""
 
+import os
 import sys
 
 
@@ -20,3 +21,123 @@ def color(text: str, *codes) -> str:
     if not sys.stdout.isatty():
         return text
     return "".join(codes) + text + Colors.RESET
+
+
+# =============================================================================
+# Terminal background detection (light vs dark)
+# =============================================================================
+
+
+def _detect_via_colorfgbg() -> str:
+    """Check the COLORFGBG environment variable.
+
+    Some terminals (rxvt, xterm, iTerm2) set COLORFGBG to ``<fg>;<bg>``
+    where bg >= 8 usually means a dark background.
+    Returns "light", "dark", or "unknown".
+    """
+    val = os.environ.get("COLORFGBG", "")
+    if not val:
+        return "unknown"
+    parts = val.split(";")
+    try:
+        bg = int(parts[-1])
+    except (ValueError, IndexError):
+        return "unknown"
+    # Standard terminal colors 0-6 are dark, 7+ are light.
+    # bg < 7 → dark background; bg >= 7 → light background.
+    if bg >= 7:
+        return "light"
+    return "dark"
+
+
+def _detect_via_macos_appearance() -> str:
+    """Check macOS AppleInterfaceStyle via ``defaults read``.
+
+    Returns "light", "dark", or "unknown".
+    """
+    if sys.platform != "darwin":
+        return "unknown"
+    try:
+        import subprocess
+        result = subprocess.run(
+            ["defaults", "read", "-g", "AppleInterfaceStyle"],
+            capture_output=True, text=True, timeout=2,
+        )
+        if result.returncode == 0 and "dark" in result.stdout.lower():
+            return "dark"
+        # If the key doesn't exist, macOS is in light mode.
+        return "light"
+    except Exception:
+        return "unknown"
+
+
+def _detect_via_osc11() -> str:
+    """Query the terminal background colour via the OSC 11 escape sequence.
+
+    Writes ``\\e]11;?\\a`` and reads the response to determine luminance.
+    Only works when stdin/stdout are connected to a real TTY (not piped).
+    Returns "light", "dark", or "unknown".
+    """
+    if sys.platform == "win32":
+        return "unknown"
+    if not (sys.stdin.isatty() and sys.stdout.isatty()):
+        return "unknown"
+    try:
+        import select
+        import termios
+        import tty
+
+        fd = sys.stdin.fileno()
+        old_attrs = termios.tcgetattr(fd)
+        try:
+            tty.setraw(fd)
+            # Send OSC 11 query
+            sys.stdout.write("\x1b]11;?\x07")
+            sys.stdout.flush()
+            # Wait briefly for response
+            if not select.select([fd], [], [], 0.1)[0]:
+                return "unknown"
+            response = b""
+            while select.select([fd], [], [], 0.05)[0]:
+                response += os.read(fd, 128)
+        finally:
+            termios.tcsetattr(fd, termios.TCSADRAIN, old_attrs)
+
+        # Parse response: \x1b]11;rgb:RRRR/GGGG/BBBB\x07  (or \x1b\\)
+        text = response.decode("latin-1", errors="replace")
+        if "rgb:" not in text:
+            return "unknown"
+        rgb_part = text.split("rgb:")[-1].split("\x07")[0].split("\x1b")[0]
+        channels = rgb_part.split("/")
+        if len(channels) < 3:
+            return "unknown"
+        # Each channel is 2 or 4 hex digits; normalise to 0-255
+        vals = []
+        for ch in channels[:3]:
+            ch = ch.strip()
+            if len(ch) <= 2:
+                vals.append(int(ch, 16))
+            else:
+                vals.append(int(ch[:2], 16))  # take high byte
+        # Perceived luminance (ITU-R BT.601)
+        luminance = 0.299 * vals[0] + 0.587 * vals[1] + 0.114 * vals[2]
+        return "light" if luminance > 128 else "dark"
+    except Exception:
+        return "unknown"
+
+
+def detect_terminal_background() -> str:
+    """Detect whether the terminal has a light or dark background.
+
+    Tries three strategies in order:
+    1. COLORFGBG environment variable
+    2. macOS appearance setting
+    3. OSC 11 escape sequence query
+
+    Returns "light", "dark", or "unknown" if detection fails.
+    """
+    for detector in (_detect_via_colorfgbg, _detect_via_macos_appearance, _detect_via_osc11):
+        result = detector()
+        if result != "unknown":
+            return result
+    return "unknown"
diff --git a/hermes_cli/config.py b/hermes_cli/config.py
index ae0d9cb75f..834b8a3fcb 100644
--- a/hermes_cli/config.py
+++ b/hermes_cli/config.py
@@ -233,6 +233,7 @@ DEFAULT_CONFIG = {
         "streaming": False,
         "show_cost": False,       # Show $ cost in the status bar (off by default)
         "skin": "default",
+        "theme_mode": "auto",
     },
 
     # Privacy settings
diff --git a/hermes_cli/skin_engine.py b/hermes_cli/skin_engine.py
index 980ed8b1fb..7ef0ad4c7e 100644
--- a/hermes_cli/skin_engine.py
+++ b/hermes_cli/skin_engine.py
@@ -114,6 +114,7 @@ class SkinConfig:
     name: str
     description: str = ""
     colors: Dict[str, str] = field(default_factory=dict)
+    colors_light: Dict[str, str] = field(default_factory=dict)
     spinner: Dict[str, Any] = field(default_factory=dict)
     branding: Dict[str, str] = field(default_factory=dict)
     tool_prefix: str = "┊"
@@ -122,7 +123,12 @@ class SkinConfig:
     banner_hero: str = ""    # Rich-markup hero art (replaces HERMES_CADUCEUS)
 
     def get_color(self, key: str, fallback: str = "") -> str:
-        """Get a color value with fallback."""
+        """Get a color value with fallback.
+
+        In light theme mode, returns the light override if available.
+        """
+        if get_theme_mode() == "light" and key in self.colors_light:
+            return self.colors_light[key]
         return self.colors.get(key, fallback)
 
     def get_spinner_list(self, key: str) -> List[str]:
@@ -168,6 +174,21 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
             "session_label": "#DAA520",
             "session_border": "#8B8682",
         },
+        "colors_light": {
+            "banner_border": "#7A5A00",
+            "banner_title": "#6B4C00",
+            "banner_accent": "#7A5500",
+            "banner_dim": "#8B7355",
+            "banner_text": "#3D2B00",
+            "prompt": "#3D2B00",
+            "ui_accent": "#7A5500",
+            "ui_label": "#01579B",
+            "ui_ok": "#1B5E20",
+            "input_rule": "#7A5A00",
+            "response_border": "#6B4C00",
+            "session_label": "#5C4300",
+            "session_border": "#8B7355",
+        },
         "spinner": {
             # Empty = use hardcoded defaults in display.py
         },
@@ -201,6 +222,21 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
             "session_label": "#C7A96B",
             "session_border": "#6E584B",
         },
+        "colors_light": {
+            "banner_border": "#6B1010",
+            "banner_title": "#5C4300",
+            "banner_accent": "#8B1A1A",
+            "banner_dim": "#5C4030",
+            "banner_text": "#3A1800",
+            "prompt": "#3A1800",
+            "ui_accent": "#8B1A1A",
+            "ui_label": "#5C4300",
+            "ui_ok": "#1B5E20",
+            "input_rule": "#6B1010",
+            "response_border": "#7A1515",
+            "session_label": "#5C4300",
+            "session_border": "#5C4A3A",
+        },
         "spinner": {
             "waiting_faces": ["(⚔)", "(⛨)", "(▲)", "(<>)", "(/)"],
             "thinking_faces": ["(⚔)", "(⛨)", "(▲)", "(⌁)", "(<>)"],
@@ -265,6 +301,22 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
             "session_label": "#888888",
             "session_border": "#555555",
         },
+        "colors_light": {
+            "banner_border": "#333333",
+            "banner_title": "#222222",
+            "banner_accent": "#333333",
+            "banner_dim": "#555555",
+            "banner_text": "#333333",
+            "prompt": "#222222",
+            "ui_accent": "#333333",
+            "ui_label": "#444444",
+            "ui_ok": "#444444",
+            "ui_error": "#333333",
+            "input_rule": "#333333",
+            "response_border": "#444444",
+            "session_label": "#444444",
+            "session_border": "#666666",
+        },
         "spinner": {},
         "branding": {
             "agent_name": "Hermes Agent",
@@ -296,6 +348,21 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
             "session_label": "#7eb8f6",
             "session_border": "#4b5563",
         },
+        "colors_light": {
+            "banner_border": "#1A3A7A",
+            "banner_title": "#1A3570",
+            "banner_accent": "#1E4090",
+            "banner_dim": "#3B4555",
+            "banner_text": "#1A2A50",
+            "prompt": "#1A2A50",
+            "ui_accent": "#1A3570",
+            "ui_label": "#1E3A80",
+            "ui_ok": "#1B5E20",
+            "input_rule": "#1A3A7A",
+            "response_border": "#2A4FA0",
+            "session_label": "#1A3570",
+            "session_border": "#5A6070",
+        },
         "spinner": {},
         "branding": {
             "agent_name": "Hermes Agent",
@@ -327,6 +394,21 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
             "session_label": "#A9DFFF",
             "session_border": "#496884",
         },
+        "colors_light": {
+            "banner_border": "#0D3060",
+            "banner_title": "#0D3060",
+            "banner_accent": "#154080",
+            "banner_dim": "#2A4565",
+            "banner_text": "#0A2850",
+            "prompt": "#0A2850",
+            "ui_accent": "#0D3060",
+            "ui_label": "#0D3060",
+            "ui_ok": "#1B5E20",
+            "input_rule": "#0D3060",
+            "response_border": "#1A5090",
+            "session_label": "#0D3060",
+            "session_border": "#3A5575",
+        },
         "spinner": {
             "waiting_faces": ["(≈)", "(Ψ)", "(∿)", "(◌)", "(◠)"],
             "thinking_faces": ["(Ψ)", "(∿)", "(≈)", "(⌁)", "(◌)"],
@@ -391,6 +473,23 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
             "session_label": "#919191",
             "session_border": "#656565",
         },
+        "colors_light": {
+            "banner_border": "#666666",
+            "banner_title": "#222222",
+            "banner_accent": "#333333",
+            "banner_dim": "#555555",
+            "banner_text": "#333333",
+            "prompt": "#222222",
+            "ui_accent": "#333333",
+            "ui_label": "#444444",
+            "ui_ok": "#444444",
+            "ui_error": "#333333",
+            "ui_warn": "#444444",
+            "input_rule": "#666666",
+            "response_border": "#555555",
+            "session_label": "#444444",
+            "session_border": "#777777",
+        },
         "spinner": {
             "waiting_faces": ["(◉)", "(◌)", "(◬)", "(⬤)", "(::)"],
             "thinking_faces": ["(◉)", "(◬)", "(◌)", "(○)", "(●)"],
@@ -456,6 +555,21 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
             "session_label": "#FFD39A",
             "session_border": "#6C4724",
         },
+        "colors_light": {
+            "banner_border": "#7A3511",
+            "banner_title": "#5C2D00",
+            "banner_accent": "#8B4000",
+            "banner_dim": "#5A3A1A",
+            "banner_text": "#3A1E00",
+            "prompt": "#3A1E00",
+            "ui_accent": "#8B4000",
+            "ui_label": "#5C2D00",
+            "ui_ok": "#1B5E20",
+            "input_rule": "#7A3511",
+            "response_border": "#8B4513",
+            "session_label": "#5C2D00",
+            "session_border": "#6B5540",
+        },
         "spinner": {
             "waiting_faces": ["(✦)", "(▲)", "(◇)", "(<>)", "(🔥)"],
             "thinking_faces": ["(✦)", "(▲)", "(◇)", "(⌁)", "(🔥)"],
@@ -509,6 +623,8 @@ _BUILTIN_SKINS: Dict[str, Dict[str, Any]] = {
 
 _active_skin: Optional[SkinConfig] = None
 _active_skin_name: str = "default"
+_theme_mode: str = "auto"
+_resolved_theme_mode: Optional[str] = None
 
 
 def _skins_dir() -> Path:
@@ -536,6 +652,8 @@ def _build_skin_config(data: Dict[str, Any]) -> SkinConfig:
     default = _BUILTIN_SKINS["default"]
     colors = dict(default.get("colors", {}))
     colors.update(data.get("colors", {}))
+    colors_light = dict(default.get("colors_light", {}))
+    colors_light.update(data.get("colors_light", {}))
     spinner = dict(default.get("spinner", {}))
     spinner.update(data.get("spinner", {}))
     branding = dict(default.get("branding", {}))
@@ -545,6 +663,7 @@ def _build_skin_config(data: Dict[str, Any]) -> SkinConfig:
         name=data.get("name", "unknown"),
         description=data.get("description", ""),
         colors=colors,
+        colors_light=colors_light,
         spinner=spinner,
         branding=branding,
         tool_prefix=data.get("tool_prefix", default.get("tool_prefix", "┊")),
@@ -625,6 +744,39 @@ def get_active_skin_name() -> str:
     return _active_skin_name
 
 
+def get_theme_mode() -> str:
+    """Return the resolved theme mode: "light" or "dark".
+
+    When ``_theme_mode`` is ``"auto"``, detection is attempted once and cached.
+    If detection returns ``"unknown"``, defaults to ``"dark"``.
+    """
+    global _resolved_theme_mode
+    if _theme_mode in ("light", "dark"):
+        return _theme_mode
+    # Auto mode — detect and cache
+    if _resolved_theme_mode is None:
+        try:
+            from hermes_cli.colors import detect_terminal_background
+            detected = detect_terminal_background()
+        except Exception:
+            detected = "unknown"
+        _resolved_theme_mode = detected if detected in ("light", "dark") else "dark"
+    return _resolved_theme_mode
+
+
+def set_theme_mode(mode: str) -> None:
+    """Set the theme mode to "light", "dark", or "auto"."""
+    global _theme_mode, _resolved_theme_mode
+    _theme_mode = mode
+    # Reset cached detection so it re-runs on next get_theme_mode() if auto
+    _resolved_theme_mode = None
+
+
+def get_theme_mode_setting() -> str:
+    """Return the raw theme mode setting (may be "auto", "light", or "dark")."""
+    return _theme_mode
+
+
 def init_skin_from_config(config: dict) -> None:
     """Initialize the active skin from CLI config at startup.
 
@@ -637,6 +789,13 @@ def init_skin_from_config(config: dict) -> None:
     else:
         set_active_skin("default")
 
+    # Theme mode
+    theme_mode = display.get("theme_mode", "auto")
+    if isinstance(theme_mode, str) and theme_mode.strip():
+        set_theme_mode(theme_mode.strip())
+    else:
+        set_theme_mode("auto")
+
 
 # =============================================================================
 # Convenience helpers for CLI modules
@@ -690,6 +849,14 @@ def get_prompt_toolkit_style_overrides() -> Dict[str, str]:
     warn = skin.get_color("ui_warn", "#FF8C00")
     error = skin.get_color("ui_error", "#FF6B6B")
 
+    # Use lighter background colours for completion menus in light mode
+    if get_theme_mode() == "light":
+        menu_bg = "bg:#e8e8e8"
+        menu_sel_bg = "bg:#d0d0d0"
+    else:
+        menu_bg = "bg:#1a1a2e"
+        menu_sel_bg = "bg:#333355"
+
     return {
         "input-area": prompt,
         "placeholder": f"{dim} italic",
@@ -698,11 +865,11 @@ def get_prompt_toolkit_style_overrides() -> Dict[str, str]:
         "hint": f"{dim} italic",
         "input-rule": input_rule,
         "image-badge": f"{label} bold",
-        "completion-menu": f"bg:#1a1a2e {text}",
-        "completion-menu.completion": f"bg:#1a1a2e {text}",
-        "completion-menu.completion.current": f"bg:#333355 {title}",
-        "completion-menu.meta.completion": f"bg:#1a1a2e {dim}",
-        "completion-menu.meta.completion.current": f"bg:#333355 {label}",
+        "completion-menu": f"{menu_bg} {text}",
+        "completion-menu.completion": f"{menu_bg} {text}",
+        "completion-menu.completion.current": f"{menu_sel_bg} {title}",
+        "completion-menu.meta.completion": f"{menu_bg} {dim}",
+        "completion-menu.meta.completion.current": f"{menu_sel_bg} {label}",
         "clarify-border": input_rule,
         "clarify-title": f"{title} bold",
         "clarify-question": f"{text} bold",
diff --git a/tests/hermes_cli/test_skin_engine.py b/tests/hermes_cli/test_skin_engine.py
index 6a5a032f1c..7732007df2 100644
--- a/tests/hermes_cli/test_skin_engine.py
+++ b/tests/hermes_cli/test_skin_engine.py
@@ -13,9 +13,13 @@ def reset_skin_state():
     from hermes_cli import skin_engine
     skin_engine._active_skin = None
     skin_engine._active_skin_name = "default"
+    skin_engine._theme_mode = "auto"
+    skin_engine._resolved_theme_mode = None
     yield
     skin_engine._active_skin = None
     skin_engine._active_skin_name = "default"
+    skin_engine._theme_mode = "auto"
+    skin_engine._resolved_theme_mode = None
 
 
 class TestSkinConfig:
@@ -312,3 +316,65 @@ class TestCliBrandingHelpers:
         assert overrides["clarify-title"] == f"{skin.get_color('banner_title')} bold"
         assert overrides["sudo-prompt"] == f"{skin.get_color('ui_error')} bold"
         assert overrides["approval-title"] == f"{skin.get_color('ui_warn')} bold"
+
+
+class TestThemeMode:
+    def test_get_theme_mode_defaults_to_dark_on_unknown(self):
+        from hermes_cli.skin_engine import get_theme_mode, set_theme_mode
+
+        set_theme_mode("auto")
+        # In a test env, detection returns "unknown" → defaults to "dark"
+        with patch("hermes_cli.colors.detect_terminal_background", return_value="unknown"):
+            from hermes_cli import skin_engine
+            skin_engine._resolved_theme_mode = None  # force re-detection
+            assert get_theme_mode() == "dark"
+
+    def test_set_theme_mode_light(self):
+        from hermes_cli.skin_engine import get_theme_mode, set_theme_mode
+
+        set_theme_mode("light")
+        assert get_theme_mode() == "light"
+
+    def test_set_theme_mode_dark(self):
+        from hermes_cli.skin_engine import get_theme_mode, set_theme_mode
+
+        set_theme_mode("dark")
+        assert get_theme_mode() == "dark"
+
+    def test_get_color_respects_light_mode(self):
+        from hermes_cli.skin_engine import SkinConfig, set_theme_mode
+
+        skin = SkinConfig(
+            name="test",
+            colors={"banner_title": "#FFD700", "prompt": "#FFF8DC"},
+            colors_light={"banner_title": "#6B4C00"},
+        )
+        set_theme_mode("light")
+        assert skin.get_color("banner_title") == "#6B4C00"
+        # Key not in colors_light falls back to colors
+        assert skin.get_color("prompt") == "#FFF8DC"
+
+    def test_get_color_falls_back_in_dark_mode(self):
+        from hermes_cli.skin_engine import SkinConfig, set_theme_mode
+
+        skin = SkinConfig(
+            name="test",
+            colors={"banner_title": "#FFD700", "prompt": "#FFF8DC"},
+            colors_light={"banner_title": "#6B4C00"},
+        )
+        set_theme_mode("dark")
+        assert skin.get_color("banner_title") == "#FFD700"
+        assert skin.get_color("prompt") == "#FFF8DC"
+
+    def test_init_skin_from_config_reads_theme_mode(self):
+        from hermes_cli.skin_engine import init_skin_from_config, get_theme_mode_setting
+
+        init_skin_from_config({"display": {"skin": "default", "theme_mode": "light"}})
+        assert get_theme_mode_setting() == "light"
+
+    def test_builtin_skins_have_colors_light(self):
+        from hermes_cli.skin_engine import _BUILTIN_SKINS, _build_skin_config
+
+        for name, data in _BUILTIN_SKINS.items():
+            skin = _build_skin_config(data)
+            assert len(skin.colors_light) > 0, f"Skin '{name}' has empty colors_light"