Moodle in English

In our Moodle 3.7.9. with Adaptable-Theme 2.3.2. we experience this issue: CSS variables added to "Custom CSS & JS" in the settings section of Adaptable are ignored. While other CSS style definitions work great, the vars won't. Any suggestions?

Example CSS:

/* Custom CSS Settings
-------------------------*/
setting:customcss

/* Variable definitions */
:root {
  --color-nightblue: hsla(211, 66%, 40%, 1);
  --color-skyblue: hsla(200, 67%, 55%, 1);
}