You can change the pen (line) color on the Inspector's appearance tab. It sounds like you may have a grey pen color instead of black. You can also make the font size larger.
You might also consider using a script to move this data out of a single field into a set of related records so that you an use a portal to display this information. This allows you to include a scroll bar not possible in a check box formatted field and you can use graphic objects with "hide object when" or text objects with conditional formats to reproduce the check box appearance. You can then set them up as buttons with scripts that replicate the check box functionality.
See the "check boxes with scroll bars" examples in "Adventures in FileMaking #2 - Enhanced Value Selection" for working examples of how you might set this up.
Thanks, Phil. However, I've been looking on the FileMaker website and have had no luck in finding "Adventures in FileMaking #2 - Enhanced Value Selection". Where should I look to find it?
The download link was included in my last post. You just have to click the title. This is a personal creation of mine, it's not in any way produced or endorsed by FileMaker Inc.