Answered Problems creating new Manager Theme

I've been attempting to create a new Manager Theme for Modx 2.6.1. I've duplicated the default theme, given the folder a new name and switched it on a few times. No problem. However, if I change anything in the index.css file, even one letter in a comment, when I upload and test it, most of the font awesome icons vanish. I've cleared cache, logged out and in, tried different browsers. What's strange is if I undo the css change and re-upload it, it stays broken.

Anybody else experienced this or have any idea what's going on? Is there a secret cache outside of Core I need to clear? This feels like a cache thing but manually deleting the cache didn't change anything. Any help would be really appreciated.

Oh my stars and garters you were right! I was using Dreamweaver and it was mangling the icon rules that used Unicode PUA.
You are a gentleman and a scholar.

BTW, anyone fiddling around with manager templates, the css no longer compiles from the build SASS files. Bourbon, Neat, and Font Awesome have all updated since 2014 and dozens of mixins and variables are gone now that the build SASS depended on. I've tried to clean it up but my SASS skills are pretty non-existent.

Thanks again for the help!