Updating background color to darker Base03

I noticed the default came with Base02 as the background color for the dark theme, which is frankly a little ugly :) This updates (hopefully, if I exported and staged right) it to Base03, the default background found in say, Sellout's Emacs Solarized Dark theme. I matched it to the canonical [palette](git@github.com:andschwa/osx-terminal.app-colors-solarized.git) using the color chooser droplet thingy.