This adds a button in the top right that changes the config back to the
default theme.
Code wise, it takes the header that was previously a part of the
dashboard component and moves it to the app component, and then adds the
button there. Possibly the header should be a component of it's own, but
that's more of refactor that can happen separately I think.
The config change uses the new config API to just patch the relevant
setting.
I'm not doing an automatic reload because 1) I don't want to figure out
how to do it correctly and 2) this doesn't work reliably anyway, as
for example the current gen GUI does a reload and ends up with
connection refused as the API service is still reloading...
