explicitly set bg color for every panel

There are some themes where the current font colors end up being the same as the default background color. One example is the [Solarized](http://ethanschoonover.com/solarized) "Light" theme for iTerm2 (based on an input of `{ "foo": "bar" }`:


Note how the `"foo"` key disappears since it's the same color as the background. Change the theme (in this case to Solarized "Dark"), and it shows up:


I don't know that much about xterm color sequences (or ncurses, etc.), but one thought is that if there is any explicitly colored text in a panel, consider also coloring the background of that panel using an explicit color to avoid the "invisible text" effect like the first screenshot above.