I am assuming you create your banners yourself....have you considered putting a black border on the image?
to center it you can go to the themes folder>css>index.css
look for the spot for the header or logo ( different on themes) and play with the margin sizes listed...work off a duplicate so you can upload a good one should it mess up. I do it with some of my themes at times...and then you have the occasional theme that is stubborn and won't take the edits no matter what
as for the changing of the gradient... it seems the themes are now "all-in-one" and the sections are all in one image??? not gotten around to opening one in photoshop but you could place it into photoshop then duplicate the layer and choose multiply then play with opacity to darken it just enough...theoretically.
again always use a duplicate of whatever file you work with.