The extension will be setup like the attached screen shot (with the icons on [url removed, login to view] rather than the blue sample icons). I have rewritten 3 of Tumblr's CSS files that will need to take priority over Tumblr's default css files locally (the css files I have created need to locally overwrite the corresponding files that are on Tumblr's server). Depending on the image they click in the app drop-down, the background image in the [url removed, login to view] will need to be altered. They will also be able choose to set a custom background via URL or choose a solid color with an RGB color picker.
I have also attached a picture of how the Tumblr dashboard will look after my CSS files have been prioritized.