One of the problems of looking at your poster on the screen is that it will often be brighter than your poster will print. That’s because your computer screen is lighted from behind, and your poster is not. Lots of people also have the brightness of their screen cranked way up.
Usually it’s not a big deal, but it can get in the way of reading black text against dark colors like blue, green, and red. In the example at the right, a customer used a background that shades from light to dark, and you can see how the text gets progressively harder to read as the background gets darker.
We fixed it by making the background a solid color of light gray, and everything was easy to read, and, frankly, the poster looked better that way. I’m not a big fan of fancy backgrounds that can detract from the content or readability of the poster.
If you really want to use a dark text box, use white or yellow type so that you have good contrast between the type and the background. That makes the type easier to read, which makes you presentation more effective. The human eye likes dark type on a white page, though, and that’s why we set our templates up that way.