Some older posts will have colors not suited for our new forum's backgrounds. You can't change other peoples' text, but you can change your background in your control panel (User CP> Edit Options> Board Style). You can change between a light and dark background to see which one better suits the text you're having trouble reading.
I think having a white and a dark grey theme as the only two options is a really bad idea for exactly this reason - Any coloring in lighter or darker shades of grey for text and other designs that rely on dark or bright backgrounds will be messed up for one option or the other. Users don't have the option to make a light and dark version of their transmission bars/signatures/text formatting/other artwork like it is used with the forum logo.
At least in the old forum, both Deep Blue and Blue Solution users had no problems reading white/light-grey text, and hardly anybody was using the bright IPB default theme.