[TMCU-810] [Pure Black] earn: input surfaces and stablecoin convert hardcoded RGBA break under pure-black dark mode

Description\n\nMirrors Jira TMCU-810.\n\nPart of epic #1271 / TMCU-622.\n\nJira status: To Do\n\n### Jira details\n\n## Getting Started\n\nTo reproduce the pure-black UI issues in scope for this ticket, you need to enable the preview flag locally. It is off by default.\n\n1. Enable the flag\n\nIn your local (copy from if you don't have one), set:\n\n\n\n2. Rebuild the JS bundle\n\nThe flag is inlined at bundle time by \u2014 a Metro hot reload is not enough. Restart with a clean cache:\n\n\n\nIf you changed native code recently, also re-run Usage Error: Couldn't find a script named "start:ios".

$ yarn run [--inspect] [--inspect-brk] [-T,--top-level] [-B,--binaries-only] [--require #0] ... / Usage Error: Couldn't find a script named "start:android".

$ yarn run [--inspect] [--inspect-brk] [-T,--top-level] [-B,--binaries-only] [--require #0] ....\n\n3. Switch the app to Dark mode\n\nPure black only takes effect in dark mode (light mode is intentionally unchanged). Settings \u2192 General \u2192 Theme \u2192 Dark.\n\n4. How it works (FYI)\n\n* is the first import in and mutates before the twrnc preset bakes them into Tailwind config. This is the only reason / / etc. resolve to pure-black values.\n* Two stopgap helpers in \u2014 (returns ) and \u2014 are how existing surfaces have been elevated to avoid collapse. Prefer these over hardcoding tokens; they no-op when the flag is off, so they're safe to land before the global rollout.\n\n5. Turning it off\n\nSet (or remove the line) and Usage Error: Couldn't find a script named "watch:clean".

$ yarn run [--inspect] [--inspect-brk] [-T,--top-level] [-B,--binaries-only] [--require #0] ...\n