Visual Editor tags

Ok, so I just designed a theme for a friend’s blog, but I have run into a weird little problem, and don’t know my way around it.

So naturally I used CSS in order to stylize EVERYTHING for his blog so that I would need to alter as little php/html as possible, but now comes the issue. Whenever he uses the visual editor (and even html editor) to add content to his blog, it shows up weird. The content inherits all the tags that I previously used for other parts. How do I get around this so that what he sees in his editor is what he gets on his page without my styles interfering?