OK, first, use
ZetaBoards, not InvisionFree. It's an upgraded InvisionFree by the same company. InvisionFree is barely even supported by that company any more.
Next, go
here for your themes. If you decide to stick to InvisionFree, there's themes for that there also.
If you need any help installing those themes, ask away and I'll help you out. I've done some things to my ZetaBoards forums that many forum admins, even ones that use software like IPB, don't even know is possible.