I can't find the button that changes your theme to the OBC theme. Yes I do have Outrageous Builders' Club (OBC). I might be blind but I honestly can't find it.
Changing your theme on the ROBLOX website has sadly, been removed. This includes the OBC theme.
You can still use userstyle extensions to change the look of the website. Stylish has been recommend before, but it is now involved in some shady analytics and user data tracking by a company called SimilarWeb. There are safer alternatives out there, such as Stylus.
Roblox themes were removed, but you can get a browser plugin/extension such as Roblox+.