Custom CSS and/or light mode

I’d like the ability to have a plain text box where I can enter CSS rules that will be applied added to the existing styling. That would open up simple changes like changing the value of --primary to more complex styling for things like transforming simple tables into full-fledged character sheets, etc. The sky is the limit.

As an alternative, a light mode would at least give some of my players the option for dark text on light background if they prefer.