Changing content color

I am trying to change the color of the blog titles from dark to white as I have a black backgound. I am using the Rowlings theme and Siteorgin css where that helped me change the background to black. But when you first start wordpress.org(which I am), there is a default Hello World post, that and anything else I write about I want to show up as white for the headline so it goes against the black background. Any help would be greatly appreciated.