[TMCU-625] Create A/B testing feature flag for Pure Black Background

### Description\n\nMirrors Jira [TMCU-625](https://consensyssoftware.atlassian.net/browse/TMCU-625).\n\nPart of epic #1271 / [TMCU-622](https://consensyssoftware.atlassian.net/browse/TMCU-622).\n\n**Jira status:** Canceled\n\n### Jira details\n\n**Goal:** Implement the remote feature flag in LaunchDarkly to control the A/B test.\n\n**Key:** \n\n**Variations:**\n\n* : Current background color schema (Dark Grey).\n* : Pure Black () background schema.\n\n**Implementation Details:**\n\n* Follow for naming and configuration.\n* Ensure mobile SDK availability is enabled in LaunchDarkly.\n* Use JSON format with percentage rollout configuration.\n* Flag should be accessible via .\n\n\n