2. Download ResBuilder and install it. (Alternatively Resource Hacker, but Resbuilder has some advantages when editing pictures.)
Start Resbuilder and open C:\Windows\Resources\Themes\Luna\luna.msstyles. In the bitmap section find BLUE_STARTBUTTON_BMP and in the right pane click the Edit button to send it to your favorite graphics editor and edit it to your liking Don't replace the pink color. This one is used as a placeholder for transparent areas.. When finished click File - Save to send it back to Resbuilder. There click File again, but this time Save As. Save it as luna.msstyles to any directory other than the original one.
Open Windows Explorer, look in Windows\system32\dllcache and if present Windows\ServicePackFiles\i386 for a file called luna.mst, delete or rename it and copy your modified file in under this name.
Boot into safe mode, rename the original C:\Windows\Resources\Themes\Luna\luna.msstyles file and copy your modified file there. If there are File Protection popups answer them with cancel and yes. Reboot.