There´s a problem with dark themes: They are usually offered as an additional option for systems that have been in use for years. Now, in the recent years, authors may got the idea to set individual text and background colors. Let's say, for example, the text color for a "memory aid" is a dark-red or purple. The used colors serve a purpose as highlighting or signaling colors, but they are not contrasted enough on a dark background.
Another contrast problem is transparent graphics that depict black on a transparent background.
My idea for a session is to develop a JavaScript that identifies and resolves contrast issues.