Unfortunately, no, there are too many extensions to deny that way. You /could/ use the https://learn.microsoft.com/en-us/deployedge/microsoft-edge-policies#extensionallowedtypes policy to allow only themes to be installed, but that would conflict with your desire to allow some non-theme extensions. I've sent the request for a new theme-specific policy that would trump the extension restriction policies to the proper team. Thanks for a great idea!