Add dark theme alpha
What does this MR do?
Add experimental dark theme
This gets the bulk of necessary changes in place, and gives us a single place to make theme fixes. There are a bunch of still-broken things, a bunch of unstyled things, and the palette needs fixing. Not behind a feature flag since it's opt-in, people can change back if anything is unusably broken (I've confirmed changing themes while in dark mode works!).
There are a few minor changes, the most drastic color change is #ececec to #f0f0f0 (mainly for input-prepend background, increases contrast slightly).
Added 'Dark mode (alpha)' theme to navigation themes. Used nav themes because:
- Didn't want to add a whole other section/type of theme
- Dealing with nav themes + dark mode is extra effort (possible in future)
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Edited by Simon Knox