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:
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.