Improve 16-color fall-back colors for streams

[neiljp]

Currently these are left empty, which can result in strange behavior - in some instances I've seen this show up with invisible text?

At best, we end up with white text for # and P prefixes - but also the same (not inverted) styles for the stream colors in the message-box stream/topic bars.

If we can't degrade the colors nicely, we could at least try to:

• Show some color for each stream (and inverted for the bars)
• Separate the color for private and public streams?

This should be more easily testable now that #563 is merged.