Allow switching from system theme

[tlmii]

Currently the dashboard always follows the system theme, but that's not necessarily what everyone wants. This PR does a couple of things:

1. Adds a small settings dialog with these options for theme: System, Light, Dark
2. Modifies the current theme immediately based on the selection in that dialog
3. Sets a cookie based on the selection so that the setting is respected the next time it is launched rather than just defaulting to system theme.
4. Fixes a few small issues with the dark theme in dialogs after Make dark theme darker #297

[tlmii]

Screen recording:

[dbreshears]

Does it make sense to push to the bottom left?

[davidfowl]

Or top right?

[tlmii]

Went with top right because the alignment there was more straightforward than getting a nav menu link at the bottom: