The client
folder is not part of the installed Wagtail package - instead, the package contains compiled and minified versions of the CSS and JavaScript assets as part of the Django apps that make up Wagtail, for example under wagtail/admin/static/wagtailadmin/css/
. This is because Wagtail's stylesheets are written in Sass and need to be compiled using a Node / NPM-based toolchain to be usable in the browser, and end users of Wagtail aren't expected to have this toolchain installed.
As a result, making arbitrary customisations to Wagtail's CSS isn't formally supported - to do it, you'll need to start from a checkout from Github, and set up a dev environment as per the documentation. As of Wagtail 2.12 it is possible to customise the colour scheme using CSS variables, though.