DEV: Live refresh all themes when watching stylesheets (PR #10337)

While working on multiple themes concurrently (multiple browsers open, each set to a different theme), I was frustrated by styles only refreshing on one browser, the one that was loaded last.

This changes triggers SCSS refreshing for all enabled themes, thus making it easier to see changes that affect multiple themes locally. Refreshes are slightly slower given the multiple messages, but I think we should try it, it is nice to have all themes update.