In short: It is not possible to do that in HTML or CSS.
HTML isn't a programming language. It's a markup language, defining how browsers should render your page. The same goes for CSS. It defines how elements with a certain class or id should be rendered, but they can't do if ... else ... tests like a programming language would.
What can you do?
You can use javascript. With javascript you can check which page you're on, and based on this result, you can either change the text contents, or change the classes. By changing the classes of an element, you could activate or deactivate certain css rules as you wish. It's fairly easy to implement this in javascript, as you'll find lot's of examples on the internet, and for example jQuery can make it easier for you to alter the page contents.
To get the current page in javascript, see this answer:
Get current URL in web browser
Another alternative would be using a server-side programming language like PHP. You could change the header depending on which page was requested. However, this might be a little overkill if you only need to change the header.