Thanks for this! However I wish it was a darker gray
Step 1: find the Surface Picture Browse to C:\Windows\Web\Wallpaper\Surface and open up Surface.jpg.
Step 2: modify it It will open in the Modern UI App by default. Touch it to bring up the bottom bar and click "Edit". Touch "Light" on the left, then "Brightness" on the right. Setting it to -75 looks pretty good. Click "Save As" at the bottom and save it to your desktop or pictures folder or wherever.
Step 3: set it as your background Go back to your Desktop. Tap a blank area on the screen and choose "Personalize". Click "Desktop Background" at the bottom. Browse to the picture you just saved. Voila.
*btw, you can also use this method to make it blue or purple or red... so that it matches your keyboard. Just instead of changing the brightness, change the color instead.
*btw again, if you install the KVADPhoto+ free App (which has great reviews and i've seen recomended on several sites) you can apply all sorts of filters and make the stock photo truly unique. The "Old-Style" filter is my favorite so far.