Summary

Do not edit the CSS directly, edit the source SCSS files and the CSS files will be generated automatically when building with meson + ninja and left inside the build directory to be incorporated into the gresource XML (you'll need to have sassc installed).

How to tweak the theme

Adwaita is a complex theme, so to keep it maintainable it's written and processed in SASS, the generated CSS is then transformed into a gresource file during gtk build and used at runtime in a non-legible or editable form.

It is very likely your change will happen in the _common.scss file. That's where all the widget selectors are defined. Here's a rundown of the "supporting" stylesheets, that are unlikely to be the right place for a drive by stylesheet fix:

File	Description
_colors.scss	global color definitions. We keep the number of defined colors to a necessary minimum, most colors are derived from a handful of basics. It is an exact copy of the gtk+ counterpart. Light theme is used for the classic theme and dark is for GNOME3 shell default.
_drawing.scss	drawing helper mixings/functions to allow easier definition of widget drawing under specific context. This is why Adwaita isn't 15000 LOC.
_common.scss	actual definitions of style for each widget. This is where you are likely to add/remove your changes.

You can read about SASS on its web page. Once you make your changes to the _common.scss file, you can run ninja to generate the final CSS files.