Reference guide styling difficult to read in dark mode

[prolog]

When using DH in dark mode, the reference guide is very difficult to read - dark type on a dark background, except for a few cases (I think it's the section headers) that are black text on white.

To reproduce:

• Get the latest DH
• yarn dev, open localhost:8080 in a browser in dark mode
• Click "Reference Guide"

[ddooley]

Thanks for bringing this to our attention. It seems we should control the styling of that popup a bit better.