In reference to Xyroid's answer which has - so far - received no points, maybe this is because the link has changed / gone.
OblyTile, for those who want a GUI to manage Tile design, works perfectly on my Windows 8.1
The link to this is:
http://forum.xda-developers.com/showthread.php?t=1899865
In particular, the developer has provided much improved icons for some internal windows apps (e.g. Control Panel) where the standard tile doesn't really match the design of the other Modern App tiles.
(I would have just added a comment, but I still need more reputation)!