Give the app a dark color scheme

[dshirley]

It feels pretty bland right now. In mockups the following looked good:

dark grey background
yellow for the primary metrics (OG, FG, IBUs, etc)
green for the secondary metrics (table view cell numbers)
white for the primary text in table view cells
light grey for the secondary text in table view cells

As part of this task, it will likely be necessary to modify how the beer color is displayed on the malt additions page. Right now it commandeers the entire gauge background, which is a bit of an eye sore. A small circle with the color would probably be better. This could potentially be a task in and of itself.