It’s impossible to apply a full theme to the release version of Firefox 57+. This is one of the types of extensions which were disallowed. You can apply some CSS changes using the chrome/userChrome.css file in each profile.However, doing a full theme there would be quite complex.
The CSS files which describe the look of the Firefox UI are contained in the omni.ja files within the Firefox install directory. Each of the omni.ja files are zip-archives (the extension has been changed to
.ja) containing 1,500 to 2,700 files, or so. You would need to make a detailed analysis of what changed from FF56 to FF57 and port the portions you wanted to keep while accounting for all changes in the Firefox UI’s DOM. This is…complex. Keeping up with such changes was one of the things that made keeping a full-theme updated difficult.
It is possible to unpack the
omni.ja archives, make changes, repack them and place them back in the Firefox install directory. You’d have to do this every time that Firefox updates. Depending on the changes you’re making, this can be quite a bit of work.