The blog header is like a crown for the blog. Very important element. And if the header and the background match well together, then the blog blog looks so distinguished.
One very important thing is the right size of the header. With 2 column regular Mimima, the header width is 648 px, then the header fits perfectly inside the borders and is well centered. If your header width is something else, it might best to change the widht to 648 px with photo editing software and then install it again.
With 3 column blogs, the header widht can differ, but the header width which I use on my 3 column backgrounds is 868 px.
If your header is already quite close to right size, it's easy to change, but if your image is a much smaller, it might become blurry. Then it's better to make a new image, which is closer to right size.
An other way to make the header match better is remove the borders from the header. You must change the border width from 1px to 0px.
margin:0 auto 10px;
border:0px solid $bordercolor;
border: 0px solid $bordercolor;
Or you can of course adjust just the color of the borders from "fonts and colors". Choose the color which is the closest of the background color.
Have a wonderful Easter!