Flatten the default theme and align its colors with Codex and Bootstrap 5

[malberts]

The default theme no longer enables Bootstrap's shadows and gradients, so buttons, dropdowns, cards and the navbar render flat. Bootstrap defaults both $enable-shadows and $enable-gradients to false; Chameleon had overridden them to true since 5.x, giving the pre-Bootstrap-5 skeuomorphic look. Going flat restores Bootstrap's own default.

The accent colors are aligned with MediaWiki's Codex design tokens: primary and link use color-progressive (#36c), danger uses color-destructive (#d73333), and success uses color-success (#177860). Because the skin renders Codex and OOUI widgets inline (Special:Preferences, Special:Block, the RecentChanges filters, etc.), matching these makes the skin's own buttons and links share one color with those widgets instead of a separate Bootstrap blue. Links now follow the primary color instead of the previous separate cobalt blue.

The default theme's remaining Bootstrap 4 palette values are also updated to Bootstrap 5.3: $pink, $cyan (which also drives $info), and $dark. These were leftovers from the theme's Bootstrap-4-era copy of Bootstrap's variables that had kept their Bootstrap 4 values through the upgrade.

Heading sizes are deliberately kept at Chameleon's existing scale rather than taken to Bootstrap's larger defaults.

The colors are static values for now; referencing the Codex CSS custom properties directly, so they track at runtime and in dark mode (#449), is a possible follow-up.

Screenshots (UI, Playwright)

Special:BootstrapExamples. Before has the Bootstrap 4 palette with shadows and gradients and cobalt links; after is flat, with Codex-aligned accents and links on the primary blue.

Before	After

_{AI-authored with Claude Code, Opus 4.8 (1M context); iterative design direction from @malberts across many rounds; diff not yet human-reviewed; compiled and checked against a local MediaWiki render (before/after on Special:BootstrapExamples), CI green.}

[malberts]

Closing in favor of #503.

The color changes here were exploratory. The decision (see #503) is to keep the _light theme unchanged in 6.0 so the release stays non-breaking, and to build a proper Codex-aligned theme as a separate opt-in theme in 6.1. The findings and decisions from this PR carry into that issue.

_{Closed per discussion with @malberts. Written by Claude Code, Opus 4.8 (1M context).}