Theming - Theme selector

[bmingles]

We need to add a theme selector to the settings menu.

• Should show all themes available from default or plugins
• Should change theme realtime when selection changes
• For now we can hide it if only 1 theme is available. We can then easily enable during dev by adding plugins and keep it hidden from users while we still work through remaining theme support

DH-16020 will implement in DHE

See Don's POC for starting point
https://github.com/deephaven/web-client-ui/pull/1460/files#diff-d0402466d6aa89364526f1efba471ff5eb597b40ed81273323a5dc6c73137a89