Text highlighting color in dark mode.

[peterspbr]

• [x] I have checked the latest main branch to see if this has already been fixed
• [x] I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem:

Description of the problem: When highlighting some piece of text while in dark themes (i'm actually using the navy theme) it uses a bright yellow color that makes the text hard to read. I'm using Firefox while using the book.

Suggested fix: Use some color for highlighting in dark themes that gives more contrast between the text and the highlighting itself.

[willcrichton]

Thanks for the report. I'll pick a more appropriate color for dark themes and add this in the next release.

[peterspbr]

Thanks!