This module allows administrators to customize a theme's CSS through the browser, using a rich text editor with syntax highlighting and live preview.
The module has just one option in its configuration page: you can choose which themes can use the custom CSS feature. So, on the settings page of each theme enabled to use this feature, 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.
Check the module on Drupal.org.
This module was sponsored by Meedan.