Dim the beatmaps you've already owned in game in the official osu! beatmap listing.

1. Download the latest build in Releases.
2. Start the executable you just downloaded. If it crashes immediately, you may need to specify the path to your Songs directory from the command line like .\osu-bdi.exe --songs-dir "C:\Program Files (x86)\osu!\Songs".
3. Install the userscript from Greasy Fork or Github in your browser. A userscript manager such as Tampermonkey may be required.
4. Open the beatmap listing and you should see the effects after the page loads.