It is one line as it is minified CSS.
Depending on which CSS you want to change you will need to change in that file.
If you need to change CSS for all theme and not only for certain pages or certain condition you will change it in " theme.js " in the tail of the file.
or you can create other CSS file for the new Rules you need and link it in " theme.liquid " file before the closing tag of " Head " as attached Image.
I appreciate the help, but here's a screenshot of all my available Assets: (the first 2, "cosminel.css.liquid and cosmin.css" are made by me and they're empty.
As you can see there is no "style.css.liquid" in the "Express" Theme. This is not Debut Theme.