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
* Enable custom themes to theme Compound
* Remove the now redundant username color variables
They are replaced by the Compound theming options (specifically, username colors can be themed by changing the color of Compound's decorative color tokens).
* Update our cancel icon
The cancel icon we're using in the app has drifted out of sync with the ones used in our designs. We also had two identical-looking icons, so this consolidates them into one.
I've simultaneously updated our chevron icons, since in the case of the 'jump to unread' timeline button, it became clear that the weight of the new close icon did not match the thinner chevron.
* Don't squish bottom/top-aligned tooltips near the edge of the screen
* Close the timeline panel when returning to the fullscreen timeline view
* Add layout switching capabilities to ElementCall
* Bring the room header in line with the group call designs
* Bring the PiP header in line with the group call designs
* Fix lints
* Clarify tooltip CSS calculations
* Test PipView
* Expand RoomHeader test coverage
* Test PipView more
