Find your stylesheet.css file in your local copy of Zen Cart's files, and in a good text editor (not a word processor) enter this at the bottom of the stylesheet...

#cartsummaryContent { color: #000000; }

That will make the text black in that sidebox only. (Those are zeros, not ohs.)

Save the stylesheet and ftp it into your css folder, preferably in your custom folder structure, which you can read about in the Tutorials/FAQ section (link at the top of this page.)

Rob

PS
If your stylesheet has already been customized, don't make this change in the default stylesheet and then ftp it over your custom one. That would be bad. ;-) Download a copy of your custom one and edit it, then ftp it back to the server.