Change community header background for each community

Hello,

Our community overview headers default to https://mentor.jiveon.com/8.0.0.ece8576/images/placeBackgrounds/bokeh1.png.

I would like to replace this on each and every community with a custom color in one foul swoop.

Possible?

Is it also possible to change the default template applied in community overviews to a custom one I built?

Thanks.

Ronan