The paired-passage tabs stayed white with black ink on every color scheme while the
passage body beneath them went dark, because the tab fill, the tab ink and the selection
indicator came from MUI's palette (`theme.palette.common.white` / `.black`), which does not
follow `--pie-*`. They now read `color.background()` and `color.text()` from
`@pie-lib/render-ui`, which resolve to `var(--pie-background, ...)` and
`var(--pie-text, ...)` -- the same tokens the passage body already uses, so the tab now
carries the colour of the panel it opens.

The two `#D9DADA` borders -- the tab outline and the strip's bottom rule -- become
`color.borderGray()`. The literal measured roughly 1.2:1 against the dark schemes'
surfaces, and the tab outline is the only thing separating an unselected tab from the strip,
so it was effectively absent exactly where it did the most work. `--pie-border-gray` is
stepped to the 3:1 non-text minimum in every built-in scheme.

The strip behind the tabs keeps a host-supplied `--pie-passage-header-background`
unchanged on every scheme, which is deliberate: Knowledge Checks' pale green-blue is a
client decision and not a scheme's business to override. What changes is the no-host
fallback, from a white literal to `color.backgroundDark()`, so the strip follows the theme
instead of staying white over a dark body. That covers hosts which never opt in, including
the section player when its own card-header token is unset.

Both `theme.palette.common.*` lines carried "replace with color.background() once PD-2801
is DONE". PD-2801 is about adding background styling to OT's `color-contrast` class -- the
mechanism pie-theme superseded -- and has been Blocked since 2023, so it no longer gates
this.

Not changed: the selected indicator stays `color.tertiary()` and the MUI indicator bar
stays `color.white()`. Both already resolve through tokens every scheme sets, `--pie-white`
being remapped to the scheme's own background rather than to white.