I'd like to install a theme on only one site of a multi-site network install. Is there a way to do that?
Yes, you can enable a theme for a single site (as opposed to enabling it for your entire Multisite network).
Go to the site's Edit Site backend page and select the Themes tab. The URL should be something like
example.com/wp-admin/network/site-themes.php?id=[site ID]. You'll need Super Admin privileges on the Multisite network.
Enable the theme(s) you'd like to allow on the site. Once that's done, administrators in the site will be able to use the selected theme(s).
Edited to add: You shouldn't need to know the site's ID to do this. Once you're logged in, go to the site's Dashboard. You'll see the site's name in the top bar. Mouse over it and select Edit Site from the dropdown:
Then select the Themes tab: