Spreadsheet in Guide/Extra Topics/Reactivity in Depth not readable in Dark Mode

[c0lt]

I don't know if this issue is in the right place here or should go in the theme repo.
The Spreadsheet in the Reactivity in Depth page is not readable in Dark Mode. The contrast of the headers is too low and the content of a selected cell can't be seen at all:

[skirtles-code]

I believe this is fixed by #1564.