Content boxes in dark mode

[fmaussion]

Describe the bug

In current jupyter book, the content box titles are difficult to read in dark mode:

Reproduce the bug

Got to: https://jupyterbook.org/en/stable/intro.html

Switch to dark mode.

List your environment

No response

[tuhe]

I think a related issue which is tabs in dark mode. An example:

On some monitors the gray-in-white text can be very difficult to see. If we could just keep the outline of the tab I think that would be much better!

Link: https://cp.pages.compute.dtu.dk/02002public/

[wragge]

Similarly, the expand links you get when you use the hide-input tag are difficult to read in dark mode.