Prior to Windows 7-10, there was no need for one. You had whichever colour theme you liked across the OS, and most native applications took advantage of that. Something could be said for code reuse.
There are no bundled "themes" per se, but it's not very difficult to configure the colors. If you don't feel like doing that I'm sure you can find people sharing their config on the DOpus community forum.
I think there are dark themes -- see 1:00 minute mark in this demo video