None of the packaged themes with installers are going to work correctly with windows 8.1.

At best you wont be able to see anying in control panel or the like at worst they bork the boot.

The ones you merely copy and paste into the theme folder wont harm 8.1 in anyway but most of it wont work.

Fortunaly proably not much tweaking needed for the windows 8 themes is needed to work on 8.1.