Well, if the designer has specified the colour for an area of the screen and you've told your browser to ignore what the web designer chose and always use black, then clearly you won't see what the designer intended.
For example, your question is displayed in a div called "questionBox". The designer defined that as being a white area surrounded orange. But You've told your browser to ignore the colours and use black, so it's going to display as a black area surrounded by black. Naturally, the surround is now invisible.