Allows Super Admins to control which themes are visible to admins of a given site within a multi-site WordPress environment.
If you are a Super Admin, navigate to the themes page in the admin section. Each theme will have its visibility status (i.e. 'is Visible' or 'is Hidden'). Next to this is a link ('make visible' or 'make hidden'). Clicking on this link will change the status of this theme in the current site.
Navigate to the 'Themes Visibility Manager' page in the 'Super Admin' menu. From there, find the theme in question and click on 'Globally Show' or Globally Hide' as necessary. You can hide/show multiple themes at once by using the check boxes and the 'Show Globally' or 'Hide Globally' buttons at the bottom of the page.
Note: Hiding/Showing a lot of themes consumes a lot of memory and may require refreshing the page for it to work (or not work at al). If this is an issue, just select a few plugins at a time.