Assets PHP addCSS() with theme:// not working as expected

The theme that I am currently trying to extend (Clean-Blog) doesn't have the custom.css in its template files like antimatter has. So to avoid having to override the whole base.html.twig I thought I could add it in the theme PHP file through the assets interface. The code that I am using is as follows:

namespace Grav\Theme;

use Grav\Common\Theme;

class CleanBlogChild extends Theme {

    public static function getSubscribedEvents()
        {
          return [
              'onThemeInitialized' => ['onThemeInitialized', 0]
          ];
        }

    public function onThemeInitialized() {
        $this->grav['assets']->addCss('theme://css/custom.css', 5);
    }
}

if i understand this correctly it should output a link with the url as follows /user/themes/clean-blog-child/css/custom.css instead it is outputting user/themes/clean-blog-child/css/custom.css so when it is inserting on a page not at the root of the site it fails to load. For example a blog post.

This is my first foray into Grav so I might just be missing something.
Thanks,