[TMCU-982] Update extension background to base color to pure black

### Description\n\nMirrors Jira [TMCU-982](https://consensyssoftware.atlassian.net/browse/TMCU-982).\n\nPart of epic #1271 / [TMCU-622](https://consensyssoftware.atlassian.net/browse/TMCU-622).\n\n**Jira status:** In Progress\n\n### Jira details\n\n### User story\n\nAs a user, when the viewport is extended past \~420px, I want the base color to be pure black so that the UI remains visually consistent with the dark mode theme.\n\n### Context\n\nAmanda Yeoh noticed that the base color is still rendering as alternative when the viewport is extended. She provided a hack in a PR to demonstrate the intended outcome.\n\n* Amanda's PR: https://github.com/MetaMask/metamask-extension/pull/44102\n\n### Acceptance criteria\n\n* Base color is set to pure black (#000000).\n* This applies when the viewport is extended past \~420px.\n\n_Work item created in Slack from a_ [_message_](https://consensys.slack.com/archives/C02VD8QG0LT/p1783009607167229?thread_ts=1782997745.906819&cid=C02VD8QG0LT)_._\n