"Youlag Theme for FreshRSS" provides a video-focused browsing experience for your RSS subscriptions. It tries to provide a similar experience to YouTube, primarily through its layout using CSS, but also a little bit of Javascript.
Git repo for more details: https://github.com/civilblur/youlag
In case you're not aware, "FreshRSS is a self-hosted RSS feed aggregator".
The idea is that you subscribe to content creators through YouTube's RSS feed https://www.youtube.com/feeds/videos.xml?channel_id={id_goes_here}, then browse, watch, save to playlist ("label"), right within FreshRSS.
There's also third-party extension for FreshRSS that provides the ability to use Invidious, but as we all know, the public instances are unfortunately struggling quite a bit as of writing.
Does it apply it to all feeds? Or can it detect what feeds are actually Youtube ones?
It applies the theme across the entire instance!
I believe it could be done however, but it's likely more suitable as an extension, opposed to a "theme" that relies on client-side css/js. I haven't explored the documentations for extensions as I intended this to be a "quick" solution to get a youtube-like experience.