Well, I finally did it — I’ve upgraded to El Capitan.
One of my big motivations was to have access to the JXA debug tool in Safari.
But I found the Safari Web Inspector font to be way too small for me — very hard to read.
After much research, I finally came across this process to change the Web Inspector font, but I had to adjust it for Safari 9.1 (11601.5.17.1) on OSX 10.11.4.
While AppleScript is a comfortable old friend, JavaScript for Automation (JXA) offers some features not directly available to AppleScript. One huge feature missing from the Apple Script Editor, is any real debugging tools. Now, with El Capitan, we have access to the Safari JavaScript Debugger and Console.
I really appreciate your working this out. One problem: the entire contents of Main.css — except for the initial comments — are on one line. Doesn't look like there is an easy way to temporarily change that then change it back. (It's not a question of which line endings are used.) What did you do? (And whatever it was, you should add it to your instructions.)
I did that so long ago I barely remember it, so I can't be of much help now.
You'll just have to do the ole "trial-and-error" method until you find something that works.
All I can say is that the approach I described in my Gist worked for me at the time.