dynamic css file for admin / backend and get_option results in Uncaught Error: Call to undefined function get_option()

I try to style the admin area with a dynamic css. I need to configure some elements regarding the frontend user defines. He can choose a color and this color should reflected in some elements in the admin area (e.g. buttons in admin area).
My approach: Setting up a evacolor.css, but let apache interpret this as a php file. This works so fare:

editor-style.css Functionality

Scenario: I have a client who has difficulty understanding that the font styling he sees in the WordPress dashboard for his posts is not necessarily the styling that will show on the front site. Overuse of the TinyMCE plugin, has resulted in every text block tag having a span inside it, as he attempts to make the editor match what is already happening on the front side because of a child theme. The text formatting is not uniform on the site because of this practice.