A blur theme for youtube (may have unintended behaviour on white theme)
Download an extension for your browser capable of injecting css into youtube, and if possible, put https://github.pato05mc.tk/youtube-blur-theme/style.css as stylesheet.
If your extension only offers the possibility to put custom css, you can use the @import css directive.
If the blur effect is not working, you might need to enable the backdrop-filter css directive, on Firefox, you can do that by going into about:config
and by enabling the layout.css.backdrop-filter.enabled flag.