Add theme support for terminal

- Make two icons on terminal page: one for light theme, one for dark theme
- When dark theme is selected, a `.dark_theme` class is applied to the `mainContainer` element which overrides colors for the terminal (colors can be easily tweaked in CSS)
- Dark theme deviates from previous dark color palette - old one was a blueish gray. Now it's more dark gray to fit rest of theme.
- Added cookie logic to remember preference for dark/light theme

Pictures: 
Those two little icons:
![icons](https://f.cloud.github.com/assets/3904767/2122709/6829b4b6-9218-11e3-93cb-9063a2387437.PNG)

documenting changes in dark color palette:
old:
![old](https://f.cloud.github.com/assets/3904767/2122710/70272554-9218-11e3-91e3-782da9279b95.PNG)
new
![new](https://f.cloud.github.com/assets/3904767/2122715/a47dc128-9218-11e3-9ca2-d57ec27a7ffe.PNG)