Hello,
It is very easy to manage/change cPanel Theme from the WHM control panel. Please follow these steps to change this.
1) Login to WHM.
2) Select "Account Functions"
3) Select "Modify an Account"
4) Select an account which you want to change Theme
5) Select "Modify"
6) Select "cPanel Theme" to "rvskin" <choose which one you like>
Now, you have successfully changed cPanel user's Theme.
Thank you,