[TMCU-987] Post-prod cleanup: fold pureBlackDarkTheme overrides into darkTheme and remove provisional utilities

### Description\n\nMirrors Jira [TMCU-987](https://consensyssoftware.atlassian.net/browse/TMCU-987).\n\nPart of epic #1271 / [TMCU-622](https://consensyssoftware.atlassian.net/browse/TMCU-622).\n\n**Jira status:** To Do\n\n### Jira details\n\n## Context\n\n was introduced as a temporary parallel theme object to allow safe preview and rollout of pure-black tokens without mutating for all consumers. Once the feature flag ships to 100% of prod users, the overrides should be promoted into directly and all provisional scaffolding removed.\n\n## Files to remove / change\n\n* \u2014 entire directory (colorOverrides.ts, pureBlackDarkTheme.ts, index.ts)\n* and its test\n* and its test (only used for pureBlackDarkTheme)\n* \u2014 remove export\n* \u2014 remove and exports\n* \u2014 fold its values into directly\n* \u2014 remove the import\n* CSS selectors: remove all rules after folding values into the base dark rule\n\n## Acceptance criteria\n\n* is \n* , , and are not exported from \n* no longer exists as a separate file\n* All existing tests pass; no coverage \n