In Backdrop CMS, you can point to a content.min.css file, assuming you’ve set Dark Mode:
/modules/tinymce/libraries/tinymce/js/tinymce/skins/content/dark/content.min.css
Toss the path in there:
Edit the background-color code in the first line in content.min.jss – body{background-color:#414141; (…)
Now it’s matching with my other forms:
Hope it works.