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:

Bexe89p

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:

Vhc Fvmi

Hope it works.