Allow to disable dark mode of App

[loichuder]

While working to disable the dark mode in daiquiri, I had the thought that if the switch to dark mode is sometimes undesirable, there should be a way to disable it.

This would avoid the CSS hack in jupyterlab-h5web.

Here is a proposal based on a disableDarkMode prop of App that gets translated in a allow-dark-mode CSS attribute so that CSS dark mode rules can be filtered. I am not convinced this is the best solution so I am drafting it for discussion.