diff --git a/ui-tui/src/__tests__/theme.test.ts b/ui-tui/src/__tests__/theme.test.ts
index db2b1eac38..b73251188f 100644
--- a/ui-tui/src/__tests__/theme.test.ts
+++ b/ui-tui/src/__tests__/theme.test.ts
@@ -1,46 +1,90 @@
-import { describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 
-import { DARK_THEME, DEFAULT_THEME, detectLightMode, fromSkin, LIGHT_THEME } from '../theme.js'
+// `theme.js` reads `process.env` at module-load to compute DEFAULT_THEME,
+// and `fromSkin` closes over DEFAULT_THEME.  A developer shell with
+// HERMES_TUI_THEME=light (or HERMES_TUI_BACKGROUND set to something
+// bright) would flip the base and turn these assertions into a local-
+// only failure.  We sterilize the relevant env vars + dynamically
+// import the module fresh so EVERY symbol that closes over the env
+// (DEFAULT_THEME, DARK_THEME, LIGHT_THEME, fromSkin) is loaded against
+// a known-empty environment.
+//
+// `detectLightMode` takes env as an explicit arg, so it's safe to import
+// statically — but we stay consistent and dynamic-import it too.
+const RELEVANT_ENV = [
+  'HERMES_TUI_LIGHT',
+  'HERMES_TUI_THEME',
+  'HERMES_TUI_BACKGROUND',
+  'COLORFGBG',
+  'TERM_PROGRAM',
+] as const
+
+async function importThemeWithCleanEnv() {
+  for (const key of RELEVANT_ENV) {
+    vi.stubEnv(key, '')
+  }
+  vi.resetModules()
+  return import('../theme.js')
+}
+
+afterEach(() => {
+  vi.unstubAllEnvs()
+  vi.resetModules()
+})
 
 describe('DEFAULT_THEME', () => {
-  it('has brand defaults', () => {
+  it('has brand defaults', async () => {
+    const { DEFAULT_THEME } = await importThemeWithCleanEnv()
+
     expect(DEFAULT_THEME.brand.name).toBe('Hermes Agent')
     expect(DEFAULT_THEME.brand.prompt).toBe('❯')
     expect(DEFAULT_THEME.brand.tool).toBe('┊')
   })
 
-  it('has color palette', () => {
+  it('has color palette', async () => {
+    const { DEFAULT_THEME } = await importThemeWithCleanEnv()
+
     expect(DEFAULT_THEME.color.gold).toBe('#FFD700')
     expect(DEFAULT_THEME.color.error).toBe('#ef5350')
   })
 })
 
 describe('LIGHT_THEME', () => {
-  it('avoids bright-yellow accents unreadable on white backgrounds (#11300)', () => {
+  it('avoids bright-yellow accents unreadable on white backgrounds (#11300)', async () => {
+    const { LIGHT_THEME } = await importThemeWithCleanEnv()
+
     expect(LIGHT_THEME.color.gold).not.toBe('#FFD700')
     expect(LIGHT_THEME.color.amber).not.toBe('#FFBF00')
     expect(LIGHT_THEME.color.dim).not.toBe('#B8860B')
     expect(LIGHT_THEME.color.statusWarn).not.toBe('#FFD700')
   })
 
-  it('keeps the same shape as DARK_THEME', () => {
+  it('keeps the same shape as DARK_THEME', async () => {
+    const { DARK_THEME, LIGHT_THEME } = await importThemeWithCleanEnv()
+
     expect(Object.keys(LIGHT_THEME.color).sort()).toEqual(Object.keys(DARK_THEME.color).sort())
     expect(LIGHT_THEME.brand).toEqual(DARK_THEME.brand)
   })
 })
 
 describe('DEFAULT_THEME aliasing', () => {
-  it('defaults to DARK_THEME when nothing signals light', () => {
-    expect(DEFAULT_THEME).toBe(DARK_THEME)
+  it('defaults to DARK_THEME when nothing signals light', async () => {
+    const { DEFAULT_THEME, DARK_THEME: DARK } = await importThemeWithCleanEnv()
+
+    expect(DEFAULT_THEME).toBe(DARK)
   })
 })
 
 describe('detectLightMode', () => {
-  it('returns false on empty env', () => {
+  it('returns false on empty env', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+
     expect(detectLightMode({})).toBe(false)
   })
 
-  it('honors HERMES_TUI_LIGHT on/off', () => {
+  it('honors HERMES_TUI_LIGHT on/off', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+
     expect(detectLightMode({ HERMES_TUI_LIGHT: '1' })).toBe(true)
     expect(detectLightMode({ HERMES_TUI_LIGHT: 'true' })).toBe(true)
     expect(detectLightMode({ HERMES_TUI_LIGHT: 'on' })).toBe(true)
@@ -48,7 +92,9 @@ describe('detectLightMode', () => {
     expect(detectLightMode({ HERMES_TUI_LIGHT: 'off' })).toBe(false)
   })
 
-  it('sniffs COLORFGBG bg slots 7 and 15 as light (#11300)', () => {
+  it('sniffs COLORFGBG bg slots 7 and 15 as light (#11300)', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+
     expect(detectLightMode({ COLORFGBG: '0;15' })).toBe(true)
     expect(detectLightMode({ COLORFGBG: '0;default;15' })).toBe(true)
     expect(detectLightMode({ COLORFGBG: '0;7' })).toBe(true)
@@ -56,38 +102,119 @@ describe('detectLightMode', () => {
     expect(detectLightMode({ COLORFGBG: '7;default;0' })).toBe(false)
   })
 
-  it('lets HERMES_TUI_LIGHT=0 override a light COLORFGBG', () => {
+  it('falls through on malformed COLORFGBG with empty/non-numeric trailing field', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+    // `Number('')` is 0, so `'15;'` would have been read as bg=0
+    // (authoritative dark) and incorrectly blocked TERM_PROGRAM.
+    // The strict /^\d+$/ guard makes these fall through instead.
+    const allowList = new Set(['Apple_Terminal'])
+
+    expect(detectLightMode({ COLORFGBG: '15;', TERM_PROGRAM: 'Apple_Terminal' }, allowList)).toBe(true)
+    expect(detectLightMode({ COLORFGBG: 'default;default', TERM_PROGRAM: 'Apple_Terminal' }, allowList)).toBe(true)
+    // Without an allow-list match, fall-through still defaults to dark.
+    expect(detectLightMode({ COLORFGBG: '15;' })).toBe(false)
+  })
+
+  it('lets HERMES_TUI_LIGHT=0 override a light COLORFGBG', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+
     expect(detectLightMode({ COLORFGBG: '0;15', HERMES_TUI_LIGHT: '0' })).toBe(false)
   })
+
+  it('honors HERMES_TUI_THEME=light/dark as a symmetric explicit override', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+
+    expect(detectLightMode({ HERMES_TUI_THEME: 'light' })).toBe(true)
+    expect(detectLightMode({ HERMES_TUI_THEME: 'dark' })).toBe(false)
+    expect(detectLightMode({ COLORFGBG: '0;15', HERMES_TUI_THEME: 'dark' })).toBe(false)
+    expect(detectLightMode({ COLORFGBG: '15;0', HERMES_TUI_THEME: 'light' })).toBe(true)
+  })
+
+  it('uses HERMES_TUI_BACKGROUND luminance when COLORFGBG is missing', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#ffffff' })).toBe(true)
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#000000' })).toBe(false)
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#1e1e1e' })).toBe(false)
+    // Three-char hex normalises like CSS.
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#fff' })).toBe(true)
+    // Garbage falls through to the default-dark path.
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: 'not-a-colour' })).toBe(false)
+  })
+
+  it('rejects partially-invalid hex instead of silently truncating', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+    // `parseInt('fffgff'.slice(2,4), 16)` would return 15 — the strict
+    // regex must reject these inputs so they fall through to default-
+    // dark instead of producing a false-positive light reading.
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#fffgff' })).toBe(false)
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: 'ffggff' })).toBe(false)
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#xyz' })).toBe(false)
+    // Wrong length also rejected (no implicit padding/truncation).
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#fffff' })).toBe(false)
+    expect(detectLightMode({ HERMES_TUI_BACKGROUND: '#fffffff' })).toBe(false)
+  })
+
+  it('treats COLORFGBG as authoritative when present so it dominates the TERM_PROGRAM allow-list', async () => {
+    const { detectLightMode } = await importThemeWithCleanEnv()
+    // Inject a light-default allow-list so the precedence test is
+    // meaningful even though the production allow-list is empty.
+    const allowList = new Set(['Apple_Terminal'])
+
+    // Sanity: the allow-list alone WOULD turn this terminal light.
+    expect(detectLightMode({ TERM_PROGRAM: 'Apple_Terminal' }, allowList)).toBe(true)
+
+    // Dark COLORFGBG must beat the allow-list.
+    expect(
+      detectLightMode({ COLORFGBG: '15;0', TERM_PROGRAM: 'Apple_Terminal' }, allowList),
+    ).toBe(false)
+  })
 })
 
 describe('fromSkin', () => {
-  it('overrides banner colors', () => {
+  // `fromSkin` closes over DEFAULT_THEME (which is env-derived), so we
+  // must dynamic-import it after sterilizing env — otherwise an ambient
+  // HERMES_TUI_THEME=light would flip the base palette and make these
+  // assertions order-dependent on the developer's shell.
+
+  it('overrides banner colors', async () => {
+    const { fromSkin } = await importThemeWithCleanEnv()
+
     expect(fromSkin({ banner_title: '#FF0000' }, {}).color.gold).toBe('#FF0000')
   })
 
-  it('preserves unset colors', () => {
+  it('preserves unset colors', async () => {
+    const { DEFAULT_THEME, fromSkin } = await importThemeWithCleanEnv()
+
     expect(fromSkin({ banner_title: '#FF0000' }, {}).color.amber).toBe(DEFAULT_THEME.color.amber)
   })
 
-  it('overrides branding', () => {
+  it('overrides branding', async () => {
+    const { fromSkin } = await importThemeWithCleanEnv()
     const { brand } = fromSkin({}, { agent_name: 'TestBot', prompt_symbol: '$' })
+
     expect(brand.name).toBe('TestBot')
     expect(brand.prompt).toBe('$')
   })
 
-  it('defaults for empty skin', () => {
+  it('defaults for empty skin', async () => {
+    const { DEFAULT_THEME, fromSkin } = await importThemeWithCleanEnv()
+
     expect(fromSkin({}, {}).color).toEqual(DEFAULT_THEME.color)
     expect(fromSkin({}, {}).brand.icon).toBe(DEFAULT_THEME.brand.icon)
   })
 
-  it('passes banner logo/hero', () => {
+  it('passes banner logo/hero', async () => {
+    const { fromSkin } = await importThemeWithCleanEnv()
+
     expect(fromSkin({}, {}, 'LOGO', 'HERO').bannerLogo).toBe('LOGO')
     expect(fromSkin({}, {}, 'LOGO', 'HERO').bannerHero).toBe('HERO')
   })
 
-  it('maps ui_ color keys + cascades to status', () => {
+  it('maps ui_ color keys + cascades to status', async () => {
+    const { fromSkin } = await importThemeWithCleanEnv()
     const { color } = fromSkin({ ui_ok: '#008000' }, {})
+
     expect(color.ok).toBe('#008000')
     expect(color.statusGood).toBe('#008000')
   })
diff --git a/ui-tui/src/theme.ts b/ui-tui/src/theme.ts
index daeedb3377..baff80abf1 100644
--- a/ui-tui/src/theme.ts
+++ b/ui-tui/src/theme.ts
@@ -179,23 +179,129 @@ export const LIGHT_THEME: Theme = {
   bannerHero: ''
 }
 
-// Pick light vs dark. Explicit `HERMES_TUI_LIGHT` wins; otherwise sniff
-// `COLORFGBG` (set by XFCE Terminal, rxvt, Terminal.app, etc.) — last field is the
-// background ANSI index; 7/15 are the "white" slots most light themes emit (#11300).
-export function detectLightMode(env: NodeJS.ProcessEnv = process.env): boolean {
-  const explicit = (env.HERMES_TUI_LIGHT ?? '').trim().toLowerCase()
+const TRUE_RE = /^(?:1|true|yes|on)$/
+const FALSE_RE = /^(?:0|false|no|off)$/
 
-  if (/^(?:1|true|yes|on)$/.test(explicit)) {
+// Reserved for future TERM_PROGRAM-based heuristics.  Empty by default:
+// most modern terminals (Ghostty, Warp, iTerm2, Apple_Terminal) ship a
+// dark profile out of the box, so guessing wrong here is more annoying
+// than missing a light user — light users can always set
+// `HERMES_TUI_LIGHT=1` or `HERMES_TUI_THEME=light`.
+const LIGHT_DEFAULT_TERM_PROGRAMS = new Set<string>()
+
+// Best-effort RGB → luminance check.  Currently only accepts a 3- or
+// 6-digit hex value (with or without a leading `#`); the env var name
+// `HERMES_TUI_BACKGROUND` is intentionally generic so a future OSC11
+// query helper can cache its answer there too, but additional formats
+// (rgb()/hsl()/named colours) would need explicit parsing here first.
+const LUMA_LIGHT_THRESHOLD = 0.6
+
+// Strict allow-list: parseInt(..., 16) silently truncates at the first
+// non-hex character (e.g. `fffgff` would parse as `fff` and yield a
+// false-positive "white" reading), so reject anything that doesn't match
+// the canonical 3- or 6-digit shape up front.
+const HEX_3_RE = /^[0-9a-f]{3}$/
+const HEX_6_RE = /^[0-9a-f]{6}$/
+
+function backgroundLuminance(raw: string): null | number {
+  const v = raw.trim().toLowerCase()
+
+  if (!v) {
+    return null
+  }
+
+  const hex = v.startsWith('#') ? v.slice(1) : v
+  const rgb = HEX_6_RE.test(hex)
+    ? [parseInt(hex.slice(0, 2), 16), parseInt(hex.slice(2, 4), 16), parseInt(hex.slice(4, 6), 16)]
+    : HEX_3_RE.test(hex)
+      ? [parseInt(hex[0]! + hex[0]!, 16), parseInt(hex[1]! + hex[1]!, 16), parseInt(hex[2]! + hex[2]!, 16)]
+      : null
+
+  if (!rgb) {
+    return null
+  }
+
+  // Rec. 709 luma — close enough for "is this background bright".
+  return (0.2126 * rgb[0]! + 0.7152 * rgb[1]! + 0.0722 * rgb[2]!) / 255
+}
+
+// Pick light vs dark with ordered, explainable signals (#11300):
+//
+//   1. `HERMES_TUI_LIGHT` boolean — `1`/`true`/`yes`/`on` → light;
+//      `0`/`false`/`no`/`off` → dark.  Either explicit value wins
+//      regardless of any later signal.
+//   2. `HERMES_TUI_THEME` named override — `light` / `dark` win over
+//      every signal below.
+//   3. `HERMES_TUI_BACKGROUND` hex hint (3- or 6-digit) — luminance
+//      ≥ LUMA_LIGHT_THRESHOLD → light.
+//   4. `COLORFGBG` last field — XFCE / rxvt / Terminal.app emit
+//      slot 7 or 15 on light profiles; 0–15 ranges are otherwise
+//      treated as authoritatively dark so the TERM_PROGRAM
+//      allow-list below cannot override an explicit dark profile.
+//   5. `TERM_PROGRAM` light-default allow-list (currently empty).
+//
+// Anything we can't decide stays dark — the default Hermes palette
+// is the dark one.
+export function detectLightMode(
+  env: NodeJS.ProcessEnv = process.env,
+  // Injectable so tests can prove the COLORFGBG-over-TERM_PROGRAM
+  // precedence rule even though the production allow-list is empty.
+  lightDefaultTermPrograms: ReadonlySet<string> = LIGHT_DEFAULT_TERM_PROGRAMS,
+): boolean {
+  const lightFlag = (env.HERMES_TUI_LIGHT ?? '').trim().toLowerCase()
+
+  if (TRUE_RE.test(lightFlag)) {
     return true
   }
 
-  if (/^(?:0|false|no|off)$/.test(explicit)) {
+  if (FALSE_RE.test(lightFlag)) {
     return false
   }
 
-  const bg = Number((env.COLORFGBG ?? '').trim().split(';').at(-1))
+  const themeFlag = (env.HERMES_TUI_THEME ?? '').trim().toLowerCase()
 
-  return bg === 7 || bg === 15
+  if (themeFlag === 'light') {
+    return true
+  }
+
+  if (themeFlag === 'dark') {
+    return false
+  }
+
+  const bgHint = backgroundLuminance(env.HERMES_TUI_BACKGROUND ?? '')
+
+  if (bgHint !== null) {
+    return bgHint >= LUMA_LIGHT_THRESHOLD
+  }
+
+  const colorfgbg = (env.COLORFGBG ?? '').trim()
+
+  if (colorfgbg) {
+    // Validate as a decimal integer before coercing — `Number('')` is 0,
+    // so a malformed `COLORFGBG='15;'` would otherwise look like an
+    // authoritative dark slot and incorrectly block the TERM_PROGRAM
+    // allow-list.  Anything that isn't pure digits falls through.
+    const lastField = colorfgbg.split(';').at(-1) ?? ''
+
+    if (/^\d+$/.test(lastField)) {
+      const bg = Number(lastField)
+
+      if (bg === 7 || bg === 15) {
+        return true
+      }
+
+      // Slots 0–6 and 8–14 are the dark half of the 0–15 ANSI range.
+      // When COLORFGBG is set we trust it as authoritative — a non-light
+      // value here shouldn't get overridden by the TERM_PROGRAM allow-list.
+      if (bg >= 0 && bg < 16) {
+        return false
+      }
+    }
+  }
+
+  const termProgram = (env.TERM_PROGRAM ?? '').trim()
+
+  return lightDefaultTermPrograms.has(termProgram)
 }
 
 export const DEFAULT_THEME: Theme = detectLightMode() ? LIGHT_THEME : DARK_THEME