SOLVED: Hey everyone! I just experienced the same 500 error for the site.css file.
I am using Firefox Developer Edition.
I struggled to figure out the issue so I went to the console to poke around. Like some of you, I did locate this error under the Style Editor tab in Firefox's Dev Tools.
This one error broke the style sheet from compiling properly.
Essentially, the OOPS notification is an indication of where you should look for an error in your base.less stylesheet.
As you fix these errors, any additional errors present in your file will also appear.
For me, it was the extr