[6.x] Make more colors themeable

[jaygeorge]

Description of the Problem

• There were some "dark" colors in ui.css that were marked as temporary.
  • These color vars were initially added in March 2025 when we upgraded to Tailwind 4.
• Since then we've added a color system that scales different gray values, which are in turn mapped to a theme system.
• These legacy -dark- colors were scattered throughout the CP markup and are technically un-themeable, as per Some colors cannot be themed yet #13994

What this PR Does

• Replaces white with a themeable color for dark mode text
  • dark:text-white becomes dark:text-gray-50; which is almost identical but it's now a theme color
• Re-maps the temporary dark vars to the new gray, making them themeable. The values are not exactly the same but they are very very close (see the table below)
  • Gray values use the default theme (Zinc). If the CP uses a different gray palette (e.g. Stone), then shades should sync up.
• Fixes Some colors cannot be themed yet #13994

Here is how the old vars are mapped to the new gray vars

dark-* → gray-* replacement swatches

Left: original --color-dark-* hex. Right: replacement --color-gray-* (theme Zinc).

Old (dark-*)	New (gray-*)
dark-100 #dfe1e5	gray-200 #d4d4d8
dark-150 #bbbdc0	gray-300 #a1a1aa
dark-175 #93979a	gray-400 #71717a
dark-200 #5f6163	gray-500 #52525b
dark-250 #555759	gray-500 #52525b
dark-275 #515356	gray-600 #3f3f46
dark-300 #4e5157	gray-600 #3f3f46
dark-350 #43454a	gray-600 #3f3f46
dark-400 #414245	gray-600 #3f3f46
dark-500 #404143	gray-600 #3f3f46
dark-550 #3b3f41	gray-700 #27272a
dark-575 #393b40	gray-700 #27272a
dark-600 #2b2d30	gray-700 #27272a
dark-650 #242628	gray-800 #18181b
dark-700 #212223	gray-800 #18181b
dark-750 #22242a	gray-800 #18181b
dark-800 #1e1f22	gray-850 #161618
dark-900 #171717	gray-900 #0f0f10
dark-950 #161616	gray-925 #121214
dark-975 #131314	gray-950 #0a0a0b

How to Reproduce

1. Browse various components that used to use the -dark- color system. Check the "files changed" in this commit to find those.