If you go here: https://addons.mozilla.org/en-US/developers/
You have to register. You can make themes for Firefox Quantum. What I have been doing is using graphics from the WB I'm using to make the theme. Of course for myself because I don't have permission from the person who created the WB.
Once you have made the theme it's in your add-ons and you can switch the browser to the WB theme you are using.
As you can see it's not perfectly skinned. It's Black Pixels by Adni18. So far for me it's the best compromise I have found. I quit using Chrome because google is spying and harvesting my data/information and I don't like that. Edge is skinned but it wasn't very smooth on this machine. In fact Edge was slow as hell on this machine. Besides that when Microsoft switches to the Chromed version of Edge we'll be stuck with another browser that isn't skinned. I have gone with Firefox Quantum.