In Blender For Dummies, 4th edition, I use a custom theme for all of the screenshots; one that’s a little bit lighter and shows a up a bit better in grayscale print. I’ve uploaded that theme as a ‘snippet’ on GitLab. You can copy and paste the code to a text file right from that page, or directly download it using the links on the right side.
Have fun!
Edit (3 June 2020): Good news! You don’t have to download the Print-friendly theme from here anymore. As of Blender 2.83, my little theme is included with Blender by default! All you have to do is choose it from the Themes section of Preferences.