For maximum effect choose the 2015 theme on the XDA forums. To select the XDA 2015 theme (not the 2015 dark theme) scroll to the bottom of any forum page and click the white drop down box on the bottom left corner. Also ensure this stylish theme is activated by ticking it using the stylish icon in your browser toolbar.

Much of the CSS is commented out. Some is currently set not to change certain aspects of the 2015 theme but is listed so the user can easily edit to change some of the more obvious color / size / font elements in the theme.

The right side bar is removed to give extra width to the page and less vertical scrolling through forum posts. To regain the right side bar edit out the side panel entry near the top of the stylesheet.