Dark Mode isn't friendly to find

[emyller]

While in the documentation website, we get a toggle at the top bar, towards the right end. Easy and convenient!

While in the app, one needs to:

1. Click "Account" on the top right
2. Click "Account Settings"
3. Toggle "Dark Mode"
4. Find their way back to what they were doing

Acceptance criteria

• There is a Dark Mode toggle always visible in the app, preferably in a consistent UX to the docs website.
• There is an option to use system settings, and it's not called "auto".
• Navigating to tabs already open reflect the new setting immediately.