UI: Change header from black to something more Backchannel-ish

[elimisteve]

Shouldn't match the chat-going background color, but maybe a light purple?

[jimmcgaw]

It's currently the dark purple of Backchannel...but you can only tell that if you squint really hard. :)

I'm open to suggestions. I don't have a designer's eye.

[elimisteve]

Looks good for now; can always tweak later