This changes the styles of buttons, other form controls, and tabs in settings to:
1. Improve their usage of semantic colors, so that they adapt correctly to custom themes
2. Make them more visually coherent with the new Compound design system, as we integrate more of it into the app
This is a re-introduction of https://github.com/matrix-org/matrix-react-sdk/pull/12241 with the difference that we're now using our branding colors again on form colors, and buttons have become rounded to match the appearance of new Compound buttons.
Robin