Updates screenshots and link colors, adds nesting to the debug theme

Hi, I noticed the changes made to the `tests.md` file so I updated the screenshots for all themes to reflect that.

I also saw the fix from ERnsTL that changes the way links and HTML comments are handled, but the side-effect was that it changed the color of the links to the same one used for titles. I don't know if that was intentional, I assumed not and changed the color back to what it was for each theme.

Finally, I updated the debug theme (which I had forgotten) so that it now also supports nesting (e.g. italic inside bold text) and uses the same colors as before (again, maybe that was intentional). I also added a link to it to the README.