Pretty arbitrary, but I have an issue finding another Theme that properly catagorizes titles like Wii Fit or Dance Dance Revolution
I agree. Exercise should be a theme, instead of or in addition to a genre.
For example, Just Dance 2021 (https://www.igdb.com/games/just-dance-2021) is primarily a dancing game, and not all of the routines are intended to be 'exercise' routines; some of them are slower, easier, and less physically intense than others. However, because *some* physical exercise is involved, even in the slower routines, and also because some of the faster, more intense routines are designed with exercise in mind, I think that this game deserves to be assigned the theme 'exercise'. Its genres are--or should be--dance, music, and rhythm.
Another example: Kung Fu for Kinect (https://www.igdb.com/games/kung-fu-for-kinect) is primarily a fighting game, and it looks like fighting games like Street Fighter, so it belongs in the 'fighting' genre, but because players control the on-screen character by moving their bodies--using Kinect--physical exercise is a by-product of playing the game. The developers intended that to be the case, since the game's trailer mentions getting "seriously fit". So, I'm suggesting that this game has exercise as a theme. Its genre is fighting.
We can discuss whether 'exercise' should be a genre as well as a theme. Right now, I'm only suggesting that there are games that primarily belong to other genres (such as fighting) and that have exercise as a smaller component, within the boundaries of those other genres.