This module allows administrators to customize a theme's CSS through the browser, using a rich text editor with syntax highlighting and live preview.
On the settings page of each theme it will be displayed a textarea where the admin can type custom CSS code. He can choose to use a plain textarea or an editor with syntax highlighting (default). He can also preview the changes live while he types. This can be disabled as well (default is enabled). The feature can be enabled to multiple themes on the same site, and all can be lively previewed, not only the default one. The custom CSS code is injected after all other CSS.
Check this video to understand better how it works (Drupal 7 version).
Check the module on Drupal.org.
The syntax highlighting feature is provided by the CodeMirror library.
This module was originally sponsored by Meedan.