OK I've been looking around and havn't found much help so I figured I'd ask all you smart people

I have a theme called zonecoper. I recently puchased a new theme that I thought I'd like but turns out its not quite what I want. What I'd love to do is mix the two. Now i've modifyed my zone copper theme to a point. But there are bigger modifications I'd love to do but Im not sure how. For instance. instead of just making my own banner that fits in the zone copper premade banner, I'd like to remake the whole area. I have made the graphic but it's about twice the height of the one that comes with the theme. I"d also love to mess with changing the borders that go around alll the blocks and such. As far as the layout I like, I'd really just like to change and mess with the skin of it. Now I am verry familuar with photoshop and a little familuar with Dreamweaver. I thought maybe I could load up a php file in DW and view it like you would an html file but it's not quite working like I thought it might. So how just do I go about drastically changing the look of my theme and making it all still fit together and all? Any help would be verry much appreciated!