Images in docs with transparent background look bad in dark theme

Documenter defaults to OS settings for themes. Users in dark mode will see images like these, whose colors were chosen assuming a light background:



The issue names Documenter and two embedded images but no repository file or test. Start by locating the documentation configuration and checking those images in dark mode; done means the images no longer look bad against the dark background.

Written by the indexing model from the issue text.