Hi Ed. There are many examples, most of them are private communities. I tried to list a few public ones for you:
https://community.bulderbank.no/58 > Dark theme via CSS.
https://community.pipedrive.com/75 > Customized right-side navigation via HTML.
https://communaute.leboncoin.info/51 > Fully customized. Some customization is doable via CSS, some are done as custom work for this enterprise client.
https://community.deine-tierwelt.de/>; Changed the shape of the buttons and the edges via CSS.