Desktop Drape

Desktop Drape instantly blanks your computer screen, except an area of the screen you want to focus on (such as a YouTube video, a photo or a text editor). Use it to focus on your current task and make all other windows (or ads) disappear, without having to manually clean up your desktop or close any window. You can also use it to take screenshots with a clean background or check for dead (defective) pixels on LCD monitors.

http://www.freedapps.com/desktop-drape/

Context Style Switcher

Firefox extension to change page style via context menu.

http://hail2u.net/