Using a quaint cheese-grater Mac Pro with a pair of Cinema displays, and I find the label text in dialogs to be almost unreadable.
Suggestions: expose a configuration option to change the background / foreground colors in the base of the interface, expose a configuration option for minimum text size in the interface, use a different typeface, use dark on light rather than light on dark.
InDesign on the same hardware is quite readable by comparison, even though they (like you) have chosen to use a non-system interface style. I do not understand the urge to make a "cross platform" interface, rather than use standard file dialog styling from Cocoa, but that's a different bug...
Walter