Closes #47503 

I added an observer for `GlobalTheme` changes that check if the accent
colors have changed or not. I thought this was closest to the patterns
and style in the codebase. I also considered triggering a
`SettingsStore` notification on theme reload but that seemed incorrect
since the settings didn't really change, even if it would solve the
problem (as the editor already observes `SettingsStore` to detect theme
changes.

Release Notes:

- Fixed Bracket color not updating when system theme changes