Hello Deitzberg, and welcome to Seven Forums.
If you have Windows 7 Pro or above, you could use the
all users except administrators option in step 1 of OPTION TWO in the tutorial below to set Group Policy to prevent the theme from being changed by all users except administrators. See the "Related Tutorials" links at the bottom of the tutorial if you would only like to prevent specific items of theme from being changed instead.
Theme - Allow or Prevent Changing
A standard user account will not be able to make changes to the system anyways without providing the administrator's password when prompted by UAC. A standard user account can only make changes that affect their user account.
Hope this helps,
Shawn