Add styles for game index and links

This commit adds some Sass variables to use green and blue as accent
colors and to standardize padding. It also adds base styles for links.

There is a bug introduced here that I will look into where the GameIndex
doesn't update the link color when you switch the current game.

Steps to reproduce:
1) Click a game in the game index.
2) Notice that two games have the 'current-game' color in the index list
of links, even though only one has the class which should apply that
color
3) Hover on the falsely colored game, and it will go back to the normal
color