It'd be difficult from what I understand (not that i understand much). A lot of standard forums work off something called MyBB or XenForo usually or similar things. I.e they work off the same software with overlay options built in and can be customised. The Pokebase was built off Q&A software, which doesn't have the inbuilt customisation. These 'overlays' aren't overlays really but complete from the ground up designs. A lot of it's integrated with the coding.
Tl:dr off the current software it's very unlikely