I'm just this minute starting to look into that flag, but had to stop as soon as I started: it's not in the Beta (63.0b6) and I don't have latest official Firefox installed. I don't know what it being missing from the Beta could mean: either they haven't disabled AutoConfig in Beta or they've disabled it entirely are my two very best guesses. Just gonna dig around for more info on it now, I guess.
Edit: in my first swipe through Google I found the Beta release notes: https://www.mozilla.org/en-US/firefox/62.0beta/releasenotes/ which say the same thing official release notes do about the flag, so are we supposed to create the flag ourselves? The way it's worded on both pages I thought it was already baked in.
Edit 3: created the flag (New > Boolean), set it to true, created a Chrome folder and userChrome.css using these instructions: https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/, then copied/pasted the CSS from the first link in your top post into it and it worked. Immediately disabled the flag, restarted Firefox, and the userchrome.css is still working, so apparently userChrome.css isn't dependent on AutoConfig.
I was going to put off messing with Firefox internal CSS (I have a hundred linkposts I'm clearly never going to make) but wondering if userChrome.css is dependent on AutoConfig to work just blew that idea to bits. At least now I've got that answer. Now I might as well see how the add-on icon CSS works, since I'm in there anyhow.
Re: Mozilla/Firefox did it again
Edit: in my first swipe through Google I found the Beta release notes: https://www.mozilla.org/en-US/firefox/62.0beta/releasenotes/ which say the same thing official release notes do about the flag, so are we supposed to create the flag ourselves? The way it's worded on both pages I thought it was already baked in.
Edit 2: they haven't updated their official FAQ, either, to explain AutoConfig's soon to be limited to ESR, only: https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig
Edit 3: created the flag (New > Boolean), set it to true, created a Chrome folder and userChrome.css using these instructions: https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/, then copied/pasted the CSS from the first link in your top post into it and it worked. Immediately disabled the flag, restarted Firefox, and the userchrome.css is still working, so apparently userChrome.css isn't dependent on AutoConfig.
I was going to put off messing with Firefox internal CSS (I have a hundred linkposts I'm clearly never going to make) but wondering if userChrome.css is dependent on AutoConfig to work just blew that idea to bits. At least now I've got that answer. Now I might as well see how the add-on icon CSS works, since I'm in there anyhow.