Improve preferences page UX

[hmpf]

We have discussed removing the preference toggles from the user dropdown in the header and having better UX in the preferences page but we'll do that later.

Originally posted by @hmpf in #937 (comment)

Also maybe remove the toggles from the user dropdown in the header.

[hmpf]

Ideas:

• Show theme colors for each theme
• Show datetiem format rendered for each datetime format
• Don't keep the themes dropdown open after switch, or make all of the dropdowns behave the same way

[elfjes]

More idea(s):

• settings as modal instead of a separate page