I’m not sure what it is you’re trying to apply it to, but just in case you’re trying to add it to an already created theme, then that won’t work. I don’t believe you can add it to anything other than a blank template so you basically add it to your from scratch projects. Then you just open the project settings, choose it as you have above. Because it’s locked, that’s why I’m assuming you’re trying to add it something other than a blank project.