[Satan]

Open your browser of choice, so long as you can inspect the element. No I.E., you really can't do shit w/ I.E..
Once inspect element, you can use your hot key that is macro'd to your browser.

• F12 - Google Chrome
  
• Q - Firefox / Mozilla
  

Locate the global css. Once found, replace your global.css file w/ the theme that you want to rip.
Then, just mock-up the HTML from there. A strategy I use is to read the source word-for-word when I need to so I can correct any errors.

Wont work for everything, but this should fit most needs.