The Custom CSS Plugin for the Grav CMS allows to add additional CSS to your site, without editing the theme or having to create a plugin for the simple task of adding some custom lines of sylesheets.
Adds some custom CSS to your Grav site.
Open the plugin preferences in the Admin panel, and add custom CSS directly in the textarea, which will be added as inline CSS, or enter one or more relative paths to load. (e.g. /user/my_css/custom.css)