fix: save selected theme

[turjoc120]

What kind of changes does this PR include?

• Fixes or refactors
• A new challenge
• Additional documentation
• Something else

Description

closes #673

Checklist:

• All the contributions made are solely the work of me and my co-authors
• I tested the changes in this PR (if applicable)
• I added unit tests to ensure my change works (when change in Java or on front-end code)
• The PR passes pre-commit hooks and automated tests

[bendehaan]

It looks like the user default prefrence now always overrides the user setting for the site?

[commjoen]

Maybe it is better to capture this as a function with a jest test :-)?

[bendehaan]

Hi @turjoc120, are you currently working on this fix, or planning to continue work anytime soon?