Hello,
Do you mean you want to have only certain themes available to different sites in your WordPress Network?
1. First go to Network Admin > Themes
2. “Network Disable” for any themes you do NOT want to be available to all sites
3. Go to Network Admin > Sites
4. Choose “Edit” for any site you want to limit / allow themes for.
* You should get a screen with tabs across the top, such as “Info,” “Users,” “Themes,” “Settings.
5. Choose the “Themes” tab
6. Enable or Disable themes as you want for this site.
7. Repeat for any other sites.
Then, when someone is in those individual sites’ Dashboard > Appearance > Themes, only the themes you have chosen will be available for them to use.
Hope this helps!