WPGraphQL provides support for querying Themes in various ways.
Below is an example of querying a list of Themes.
Query from Authenticated User
Authenticated users with the "edit_themes" capability can query a list of Themes and see all available themes for the site.
Query from Public User
A public user or a user without "edit_themes" capability can make the same query and only the active theme will be returned.
The active theme is considered a public entity in WordPress and can be publicly accessed and WPGraphQL respects this access control right.
WPGraphQL does not currently support mutations for themes.