WPMU Dixi: How to add a custom css entry box?

How can I add a custom css entry box on the interface here: https://premium.wpmudev.org/view-screenshot/60534760_dixioptions.png ? My setup: WPMU muilt site network.

My goal is to allow each sub blog admin to overwrite any the preset css of the Dixi theme on each sub blog, respectively/individually. I know the interface above has already built in lots of useful and important styling options (thank you!). It would be great to overwrite other css that can not be changed on the interface above.

For instance, to completely change the styling of the main navigation menu, a custom css entry box would come in handy, which can overwrite anything set by the theme css.

Thanks :slight_smile: