An orange and black theme? Harlequin rasboras would work pretty good in there as well, and if you could an orange and black betta, I don't know how hard they are to find that strain but I think I've seen one before, I'll have to check. Instead of getting an RTBS you could try some of the darker cory cats or khuli loaches, and for the top of the tank maybe a flame dwarf gourami. I don't know if these are the right colors you're looking for but good luck, it should be a great tank!