Having the background of the TI-89 pages the same as your other pages helps tie your website together. And everything is readable problem when you insert a Word document directly into a post.
But if, however, you still need to post things NOT in a Word document… hmm…
Option 1. It’s quick to put the text in one of these green-background boxes.
Options 2. You could consider changing a single page to a simple single-color background. It’s a small job, so it would cost small bucks. But it’s doable.