I do not take full credit for this code. Base code comes from http://freestyler.ws/style/15341/awesome-tumblr-style by Tallain@userstyles.

1) Removes the top navigation, footer, and right column, so this style is good for those who simply post and read posts made by anyone they follow.
2) Color and style changes.
3) You can replace the header in this section. Replace the url with the url of your desired header. Make sure it's at least 300px high and the width of your screen:

#dashboard_index #header {
position: absolute !important;
top: 0px !important;
left: 0px !important;
height: 300px !important;
width: 100% !important;
background: url('http://i46.tinypic.com/sdp8au.png') center !important;
}



Here are a couple screencaps to give you an idea (they're too large to upload thumbnails here):

Before: http://img254.imageshack.us/img254/9166/tumblr1265026978367.png

After: http://yfrog.com/59tumblr1265026988098p