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.
* Use Compound primary colors for most actions
The "accent" color variable is something we should generally not use anymore: it maps to a Compound text token, which makes its semantics inappropriate for icons and backgrounds, and it clashes with the primary colors present on the Compound components we're now bringing into the app. As discussed with design, we would like to phase out its usage on interactive components in favor of the correct icon and background colors from Compound. This is a best-effort attempt at applying new colors to all the major places that we were previously using "accent": mainly, buttons and form controls are affected.
* Update some more colors
* Update test snapshots
* Fix broken screenshot
