[TMCU-994] [Pure Black] Add funds pay with modal hardcodes bg-default

### Description\n\nMirrors Jira [TMCU-994](https://consensyssoftware.atlassian.net/browse/TMCU-994).\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### Impact\n\nIn the Add funds flow, the "Pay With" BottomSheet hardcodes internal colors to . When the feature flag is true, this results in UI papercuts where the background does not match the pure black schema.\n\n### Expected behaviour\n\nThe BottomSheet background should adapt to the pure black theme (hex #000000) when the feature flag is enabled, rather than using a hardcoded dark grey/default background.\n\n### Actual behaviour\n\nThe BottomSheet background is hardcoded to , creating a visual mismatch against the pure black background.\n\n### Steps to reproduce\n\n* Start at perps home page\n* Select Add funds\n* Select Pay With asset dropdown\n\n### Environment\n\nPerps (Mobile)\n\n### Workarounds\n\nNone reported.\n\n_Work item created in Slack from a_ [_message_](https://consensys.slack.com/archives/D05SUTYLX43/p1783032906886409)_._\n