CSS frameworks can be an excellent way to add polish to your projects and avoid dealing with browser inconsistencies. There are dozens of frameworks available, but the two most popular are Bootstrap and Foundation.
In mockup.html you'll find the basic HTML structure of a page from a Reddit-like site. Explore the web sites for Bootstrap and Foundation, pick one that you'd like to try, and uncomment the corresponding stylesheet tag in the file. Then use the framework to make the page look good! You can alter the HTML or reorganize the content of the page however you like, as long as it conveys the same information. As an extra challenge, make sure the page is responsive and would look nice on mobile devices with narrow screens.