You should be able to modify the colors used by libadwaita. However, this will break dark mode, possibly accent colors, and certain apps visually, when they set their own colors for certain UI elements.
Anyway, GTK loads the CSS defined in ~/.config/gtk-4.0/gtk.css, if present. And since libadwaita defines the colors used by the UI using CSS variables, you can override the variables with the old values. Refer to the libadwaita docs for the used variables and their values.
This is also how Gradience has worked.