Hello m.r.fish,
To help us get a better understanding of the issue, we’d like to ask a few questions.
- When did this happen? Were any changes made to your device or its settings before this began?
- Does this only happen in Mail, or does it happen in other apps as well?
The above details will help us plan how to proceed.
Please also check out the information in the following link, which explains how to change the display colors on Mac to make it easier to see what’s onscreen, and how to make transparent items solid: Change display colors on Mac to make it easier to see what’s onscreen
Make transparent items solid
Some windows and areas of the desktop, such as the Dock and menu bar, appear transparent by default. You can turn these transparent areas a solid gray to make it easier to distinguish them from the background.
1. Choose Apple menu > System Settings, click Accessibility in the sidebar, then click Display on the right. (You may need to scroll down.)
2. Turn on “Reduce transparency.”
Thanks!