Post Update: This tool no longer works with newer Windows 8 versions as it was originally designed for the Windows 8 developer’s preview.
Since the inception of Windows 8, tweaking the Metro UI seems to be the top priority of Windows 8 users. Some like turning off the Metro UI, whereas, others prefer controlling the Metro user interface. Windows 8 Start Screen Editor is a portable tool for Windows 8 that enables tweaking with the Metro UI, to change the Windows 8 Metro Start Screen Background image and color. This free software is quite easy to use and provides simple options for selecting a custom solid color for the Metro UI, as well as a background image. This is option is currently not available in Windows 8 by default.
Once Windows 8 Metro Start Screen Editor is launched, it displays the default green background color with the green texture of Windows 8 (Metro) start menu. You can change the background image/color or both of these features, to customize the look of your Windows 8 Metro user interface.
To add an image to the Windows 8 Metro Start Screen, click Background Image option or select Don’t use background image if you do not wish to add a background image and merely wish to switch the background color.
Note: Please make sure that the selected background image is in PNG format and the height of the image is twice the size of your screen resolution, so that it can fill the entire Metro screen. After you have selected required options, click Patch.
This will provide you with a log off prompt, after which the changes will be applied. The below image displays the changes made to the Metro Start Screen with the help of the Windows 8 Start Screen Editor.
I would advise you to create a backup of your system before tweaking the Metro UI, as this application does not have any reset option like Windows 8 Start Tweak, which essentially performs the same function as Windows 8 Start Screen Editor.