i personally think a 2.5 is too small to decorate in a "theme", there's not enough room for theme pieces as well as the plants bettas need to be happy. you're best off going with a natural look, silk plants and rock caves and the like. if you're dead-set on having a super original tank, you should pick out the fish before the decor, then color-coordinate the two to make him "pop"
Personally if that was my tank I would find a nice textured rock, cover it in java moss (use cotton or fishing line to tie it down) and then attach a decent sized anubias to the top of it. Maybe also a couple of smaller java ferns.
This would sit in the middle of the tank and I would slope the sand so it is higher at the back then at the front. You could also maybe spread some larger chunks of gravel around the rock to make it look more natural.
I cannot stand fake plants in my tanks so this is why I suggested an alternative. Plus anubias and java moss will happily grow in very low light and require essentially no fertilisers or maintenance from you at all.