Thanks @Sommerregen for the additional info. I have thoroughly looked through the docs but maybe I missed something about theme inheritance. This also motivates me to learn more about streams because clearly they are extremely powerful ways to map paths.
I was hoping that this is the way things would work (look in child folder first then fall back to “root” user folder). Are you saying that the way to force the use of the general/shared default /user/themes and /user/plugins is to exclude the creation of these folders for the subsites? I didn’t think of that because the docs said that these folders were required, along with /pages and /config. I think what I tried was to have an empty /user/subsite/themes/ folder while keeping antimatter in the general /user/themes/ folder and Grav threw an error saying that antimatter didn’t exist.
I will try deleting the subsite themes folder tonight when I get home from work to see if that will force the fallback. Thanks for all the help!