I faced with the same issue now. I am opening TinyMCE settings.

There are extra parameters as:
Site skin (dark) and Administrator skin (dark) which have default values (white) by default.

I set values to tinymce5-dark for Set 0:

And go to create an article under Super Administrator. There is still white palette in article's edit mode:

Joomla 5.1.1, php 8.3.8

A workaround is to use the muta admin template:

I'll hope this hint could help to find and solve the dark mode issue within the atum admin template.

J5.2.0, PHP 8.2.24