With the recent update- I presume 60.0.1- I found the "Classic Theme" changes via the chrome folder no longer worked; with it in place the display is rather indescribably glitched out (elements seem to be at seemingly arbitrary positions, move around when moused over weirdly- etc.
I tried installing it from the github instructions all over with a fresh setup of Firefox 60.0.1 but was unable to get it to cooperate.
Frankly I would have just shrugged and just gone with the "default" theme/visuals, but I greatly prefer my main browser to have tabs on the bottom of the toolbar area (eg right above the content). Since after a bunch of screwing around I couldn't get that seemingly simple customization to work, I decided to try a fork of Firefox, and settled on Waterfox. Another reason to switch was because some of the recent changes they have been making to the browser seem to have a bit of a "dark design" to them; adding advertisements to the new tab page, for example, and then removing the ability to change it to point at a specific URL, is one good example of this- You have to install an add-on to change the page loaded by the new tab.
I ended up "switching" my primary browser to Waterfox, which is based on Firefox 56 currently- as a result, the ClassicThemeRestorer add-on itself still functions so I don't have to fiddle with the chrome folder at all. Of course, I still have Firefox 60 itself installed- I haven't run off into the sunset swearing or anything.