darkMode: Update string to "Dark Style"

Spawned from a discussion in a design team call and on Twitter. We should aim to stay consistent with the term “dark style”, which is the phrasing we already use in the Appearance pane in Settings, as well as my original recommendation in my call for a FreeDesktop preference.

I've avoided updating anything outside of user-facing strings here, so there's a mismatch between e.g. the file name and the string; if preferred, I can update it to be consistent.