Need write permissions to install themes on Linux Mint 17 "Rebecca" Mate

Just installed Linux Mint 17 "Rebecca" Mate. I'm trying to install a new theme. I've downloaded a couple and their instructions request that contents are copied to /usr/share/themes However, I don't have writing permissions to copy to this folder. I carried out a workaround to basically force the download into /themes, but encountered a fairly obstructive error upon booting wherein the ICE authority file could not be updated. This has since been resolved. What's a successful method?