Make themed widgets reflect the effective theme

[robintown]

So that widgets such as Element Call will show up in the right theme even if the app is set to match the system theme.

Closes #27671
Depends on matrix-org/matrix-widget-api#117

[t3chguy]

I'm not sure this is the right solution given that system theme often changes based on time (day/night), and widgets have no way to receive a theme change. Maybe a widget should just make its own media queries if there is no provided theme to match system theme and that way it can respond to theme changes.

[robintown]

Apologies for the force pushes. This is ready for another review but blocked on a related matrix-widget-api change.

[hughns]

I'm not sure this is the right solution given that system theme often changes based on time (day/night), and widgets have no way to receive a theme change. Maybe a widget should just make its own media queries if there is no provided theme to match system theme and that way it can respond to theme changes.

@robintown have these concerns been addressed?

[robintown]

Yes, they have. I'm now updating the theme dynamically with a widget action.