Theme problem

I recently made my first forum with this site, and I picked the nuclear symbol premade theme, but now i want it neutral, like a blue and white.
When i pressed a blue and white, the margins of the forum were still black, which was incredibly ugly!
I want the whole thing to be the blue and white, not keep the margins black...
How do i change this?

Also, how do i put the website banner in the correct spot once this is done?

Also, is there anyway to get a theme like that without the forumotion one?

The whole point of this is to make it look better with the blue + white theme, but also to be able to put a banner on it to make it more customized

Can someone please answer... really need help

Hello,

1) Can you please re-apply the new blue & white theme and see what that does?

2) There's really only 1 spot for the banner, which is where it currently is.
You can change the banner via:
Admin panel >> Display ( Tab ) >> Pics management > Advanced > Banner

3) Sorry, but im not following? Could you expand a bit more please?