#11 Make protocol legend background black.

[R. Ghetta]

Hello,

This patch changes the background color of the protocol legend to black. Some colors (white, light grey, yellow...) are basically unreadable on the default grey background; with a black background (like the main diagram canvas) they're much more readable.

Note that I know basically nothing about GTK; hence I'm not sure if directly re-using the style object from the background instead of creating a new one is kosher or not, but in my own testing I haven't seen it cause any problems.

Thanks,
Zev Weiss