This is a dark mode version of Newfeeder, an RSS aggregator/reader.
If hosted on your local network, e.g. via a raspberry pi, then you can set network-wide feeds to sync across all network devices that you can turn on/off separately on each device (settings are saved even after closing/re-opening browser).
Change lines 22 onwards of app.min.js to add/remove network-wide feeds.
All other feeds stored locally via cookies on a per-device basis. You you can still export/import OPML files for all your feeds just like the original Newsfeeder site. If you do end up subcribing to a bunch of feeds then I recommend export your OPML file as a backup before you delete your browser cookies!