fix(player): the Plex player's fills hold on its chrome: an amber head on a white played part

A light theme's accent as a fill on the always-dark chrome was ~2.8:1 (Slate), under the 3:1 a
control needs. A native checkbox and slider tint with `accent-chosen` (the accent elsewhere, the
media amber under `media-marks`), and so does the skip button's countdown. The progress bar plays
white with an amber head: amber there marks a place, as the intro / credits markers do, and an amber
played part made a marker ahead of the head read as already played (owner's choice of three).

docs/DESIGN.md

@@ -118,7 +118,10 @@ style guide.
   player (the Plex player, the YouTube stage) and on every surface painted `--media-chrome` (its menus,
   panels and bars), and a form there (`media-scope`) re-points `--danger` too. A chosen item there is
   white like the rest, marked by an amber tick (a track) or underline (a tab, `ui/mediaChrome`), never
-  the accent as text; a choice filled with the accent keeps it, its own text holds on it.
+  the accent as text; a choice filled with the accent keeps it, its own text holds on it. A fill that
+  must be seen there — a native checkbox or slider (`accent-chosen`), the skip button's countdown — is
+  the amber too; the Plex progress bar plays white with an amber head, so an amber marker ahead of
+  the head never reads as already played.
   `theme.contract.test.ts` measures the red on the chrome and holds every class string that paints it
   to name one of the two. Black and white are the media scrims and their text. Five schemes × dark
   and light (B2): **Midnight** (blue), **Forest** (cyan), **Slate** (fuchsia), **Matrix** (moss) each earn their