Consolidate, merge, and refactor Gin's CSS variable's into Admin theme's original variables.

Problem/Motivation

The default_admin theme (the Gin contrib theme brought into core) shipped with several overlapping CSS custom-property systems inherited from its Seven → Claro → Gin lineage: a Claro layer (--color-*, --space-*, --font-size-*, --input-*, …) and a Gin layer (--gin-*) that cross-referenced each other. Many tokens were duplicated with diverging values, a large share were dead, dark mode was driven by class-based overrides on only part of the system, and component stylesheets reached straight into raw primitive values.

The goal is a single, documented, canonical token API for the theme (and for contrib that builds on it), with first-class light/dark support.

Proposed resolution

Consolidate everything into one --admin-* namespace, organised in three tiers:

• Primitives — the raw palette (gray/blue/red ramps) and base scales. Not intended for use in components.
• Semantic tokens — the public API components consume: text, surfaces/backgrounds, borders, status families, link/primary, focus, etc.
• Component tokens — per-component values, seeded from the semantic layer.

Mode-aware tokens are expressed as a single light-dark() declaration driven by the color-scheme property, replacing the previous class-based dark-mode override blocks. The accent-color system continues to drive the --admin-color-primary* tokens.

What changed

• Split the monolithic variables file into category files under css/_variables/ (color, color-accent, spacing, typography, components, misc).
• Color: consolidated the Claro and Gin color layers into one --admin-color-* system; moved dark mode onto light-dark(); composed the semantic layer from the primitive ramps where they align.
• Spacing: one canonical spacing scale plus a density-aware scale tied to the layout-density setting; layout dimensions namespaced to avoid collision with core navigation tokens.
• Radius, typography, motion, icon sizes and misc tokens migrated to --admin-*, merging the duplicate Claro/Gin pairs.
• Routed component stylesheets off raw primitives and onto the semantic tokens.
• Fixed a class of dark-mode bugs where a fixed white was used for surfaces that should adapt.
• Pruned dead custom properties (keeping those that are live cross-module contracts).

The migration was kept value-preserving wherever practical, so the visual output is unchanged apart from a small number of deliberate, signed-off adjustments. The legacy --gin-* custom-property surface has been removed.

Remaining tasks

• Scope the component-level tokens to their component selectors rather than the global root (follow-up).
• Remove the remaining gin- prefixes from class and attribute names (separate task; also touches JS and Twig).
• Accessibility pass on the consolidated tokens (e.g. contrast review of a couple of static link/text colors in dark mode).

Notes

• Source of truth for the token definitions: core/themes/default_admin/css/_variables/. Edit the *.pcss.css sources, never the compiled *.css.
• This was accomplished with Claude code and I both doing planning, Claude doing the grunt-work, and me doing testing, cleaning up, etc.