The strips painted with `--theme-card-seed` — the raw, unmixed seed rather
than a surface token — so they read as their own band beside the sidebar and
titlebar, which share `--ui-bg-sidebar`.
Fixed at the default in `PaneTab` rather than per strip. The right-rail
preview strip never set the vars at all, so its inactive tabs fell through
to the seed even though its container was already on the sidebar surface;
correcting the fallback fixes that one for free and keeps the next strip
from regressing. With the default right, the two zone strips no longer need
to redeclare the var and paint the token directly.
f4e042f466