Store user's theme preference with temporary settings
Created by: lrhacker
Closes #36744 (closed).
Test plan
Unit test: updated the existing tests to check for the setting in local storage under the temporary settings key.
Manual test: log in and select a theme, then open an incognito tab and new browser to see that theme preference carry over (can repeat the other direction as well).
App preview:
Check out the client app preview documentation to learn more.