Note that individual "cards" (and lists) are no longer visible as "blocks", except on hover.
To change the color of the top header (green in the screenshot), just modify your board's background color, by clicking on Show Menu > Change background.

To change de card color on hover/selected, look for "card background color on hover" in the css code.

For a similar simple theme with colored top bar, see my http://freestyler.ws/style/127096/trello-theme-white-board-with-colored-header

Updates

August 2015: transparent background for crayon icon (card edit), smaller link for "Add new list", some color changes, fixed spacing problem with card edit (position of right buttons), line height of list header, updated screenshot, etc.