You'll need the Stylus extension to apply custom themes. Click on one of the theme's screenshots and copy the resulting CSS. Then, click the Stylus extension icon and Write style for: digdeeper.love
. Paste and save. Give it a name that will remind you of it. You can add all the styles I've created and switch between them at will from the Stylus menu. Sometimes I modify them, so the screenshots might not be perfect representations forever. And you will need to update your style extension to see the changes.