Copy the custom.css file to the /administrator/templates/isis/css/, and that's all, nothing more is needed.
Not everything has been redone, in particular, the calendar field and some other unobvious and rare interface elements have not been repainted.
Overrides only the styles of the Isis template, affecting only the regular Joomla extensions in the administration panel. Any third-party extensions can impose their own styles that need to be separately redefined, and this will not be included in this topic.
Main page
List view
Item view
Modal
Menu dropdown & install page
