Background Changing Color Unexpectedly

Samsung Chromebook 3 / ChromeOS version 96.0.4664.111 (Official Build) (64-bit)
K-9 Mail version 5.806
One POP3 email account

Sometimes when I read a long message in the Inbox in K-9, it starts out black text on a white background, easy to read. Then my fingers or thumb may twitch, and it sometimes turns into a purple background, making the text hard to read. What is happening? I cannot find any way to switch back to black text on a white background. How do I get around this problem?