From MozillaZine Knowledge Base
If web pages' background and text colors are displaying wrong, try checking the following settings:
- (Firefox) View -> Page Style should not be set to "No Style"
- "Tools -> Options -> Content -> Colors" - check "Allow pages to use their own colors, instead of my selections above"
- (Windows Vista and later users) Control Panel -> Appearance and Personalization -> Personalization -> Window Color & Appearance -> Appearance - uncheck "High Contrast", restart the application 
- (Windows XP users) Start Menu -> Control Panel -> Accessibility -> Display - uncheck "Use High Contrast", restart the application 
- (Windows users) Right-click the Firefox shortcut -> Properties -> Compatibility - uncheck "Run in 256 colors" and other possible Compatibility mode settings, restart the application
- (Mac users) Apple menu -> System Preferences, click Universal Access, and then click Seeing 
If none of those settings work, try it on another system. If the other system also has the same issues, it's likely a problem with the site.