Hello,
The unminified code is actually SCSS, not CSS - and there are multiple files, which ultimately get merged into theme.css
If you specifically want to remove styles from theme.css, you need to get the uncompiled SCSS code from GitHub, make the changes and compile it to CSS yourself. Here are the links for Classic and Hummingbird.
However, if you want to just add new CSS changes, you can just create a custom.css file in the assets/css directory of your theme and that should be automatically be added to all front store pages, as you can see here