You can now turn on a "high contrast" mode for forum posts in your board preferences:
Forum/ucp.php?i=prefs&mode=view
This overrides any custom font colours and font sizes that have been used in the posts, and instead uses black text on a white background, with a font size a bit bigger than standard.
We think this will make it much easier for people to settle on a single zoom level that allows them to read all posts, instead of having to adjust it where people have used smaller fonts or low-contrast colors.