Please add the ability to theme the Hasura GUI

Hey there, friends! I couldn't help but notice that you're using a Bootstrap theme to style the Hasura interface. I'm using your amazing application for some client work and showing it to a few people for the first time. One of my associates recently said to me, "I really love _everything_ about Hasura in terms of features and how easy it is, but the one thing I'd love to change is the color scheme to something darker and less prone to cause eye-strain."

I totally agree with that sentiment. Is there any chance you might be able to expose the color choices you've made into the global configuration, so that users can select this for themselves? Alternatively, even just supplying a dark theme as an alternate option would be a great compromise and tremendously appreciated. If you need extra hands to do it, we would just need to know what your pull request policy is. Thanks so much for a terrific product!

The issue names no files or tests; start by locating the Hasura GUI's Bootstrap theme and global configuration entry point. Done means users can choose a darker theme or configure the GUI's color choices, with the selected appearance visible in the interface.

Written by the indexing model from the issue text.