Indeed, the browser UI can still be customized through the use of custom "userChrome.css" and "userContent.css" files. But it has many serious downsides which, for the moment, are preventing me to definitely go this path.
I will only consider if to propose a "userChrome.css + userContent.css" combo once Mozilla will release the new theming system and, of course, once I will have these kind of files ready for public release. I already started to customize my own Firefox thanks to this workaround, but it is still a work in progress.
I would like, first, to see If the new theming system will be able or not to offer satisfying visual customization possibilities. If I see it can't offer a comparable experience as former complete themes, I may try to find a way to distribute a "userChrome.css + userContent.css" package outside the Mozilla add-ons website.
But honestly, I would really like to keep on proposing my theme as an official "add-on" on AMO website. In my opinion, it would be the best solution.
In the meantime, if you still want to keep on using my complete theme, there is still the possibility to install Firefox ESR. It will allow you to use my complete theme at least till next summer. By this time, we'll have better picture of the new theming system for Firefox Quantum.
By the way, next time, please, use the "support email" and not the review section to ask questions, as it is not the place where to do it. Thanks in advance.