Oshvam
Applying the theme will work on all visitors, only admin that has extension access can edit the Tag with Themes.
Though a nice feature is to implement permission group of which themes to display, but the extension has already went beyond with features.
Performance would be done on client, so the additional elements may induce CPU load. The color with Border will increase that load as well,
The server only outputs two data fields from server yippy-tag-with-themes.design-default(: string) and yippy-tag-with-themes.design-by-tags (json decode), if you have multiple customised themes then the json decode may have some load on the server.
Let me know if it works well for you, I'm new with flarum, so I don't have a fully populated instance to test that type of environment.