This userstyle is applied by default on a few commonly used SearXNG servers, listed at the top:

@-moz-document url-prefix("https://searx.baczek.me/"), url-prefix("https://searx.be/") {

If the theme isn't being applied to your instance, go to the userstyle's Settings > Custom included sites and add your instance to the list, for instance, *:https://searxng.example.com/*.