Hi,
There's nothing wrong with your editor. What you're seeing there is a minified version of the file, meaning the file is stripped of spaces to reduce it's file size. In short it's a good thing.
The developers at Prestashop now use a tool called webpack, which among other things, compiles multiple scss files into one file called "theme.css", which in the end gets minified to reduce file size.
The correct way to change the css would be to change the development version of the scss file and then compile everything again using webpack. You might want to do some reading about this if you want to follow this path.
However there are other ways you can do this.
One way would be by downloading the theme.css file from your server, and using a tool such as this: http://tools.bin63.com/javascript-minifier-unminifier you can upload it and unminify it, then edit it locally and once you've finished, upload it again to minify it. Once you're done upload the minified version back to your server.
Hope this helps.