stylesheet overriding other stylesheets

I had two themes installed on my website: http://midnight.whenlovefalls.com/
I also use a theme switcher so you can switch between those two.

However today I decided to install a third theme and edit that one. But after doing so I found out that the stylesheet of that theme was overriding the other stylesheets. Because my other theme’s suddenly had a yellow background in the sidebar, the titles for the post where really huge, the titles got another color and I can’t seem to change them and there is a gap between the header and navigation.

How can I stop the stylesheet from OmNomMonster to stop overriding the others