Since the release of mobile capable versions of UBB.threads there have been many stock style releases which are all clean, vibrant, and mobile friendly; a couple also use dark color schemes. These are available from the member area and are maintained by my co-developer Isaac/id242.
Myself I offer several paid styles at my homepage as well as offer Styling services for $75/hr.
In versions v7.6+ you'll be wanting something designed for newer builds as there have been significant changes to the user interface so ancient themes could have significant missing classes...
Now, forums using UBB.threads versions prior to v7.6 are not mobile capable or HTML5, so users running those builds will have less missing classes, but a significant amount of missing features compared to modern builds.
