There is a small modding community that surrounds Starcraft II outside of the official channels. These .mpq mods alter the base files to allow players to slightly customise their game client. MPQ mods are used by players to show brighter team colours, alter the backgrounds of the menus, create custom login screens, and more. With the Starcraft 1.5 update, MPQ mods will be blocked by new security protocols.
These mods are very different to the maps and new game modes that are available on the Blizzard arcade - they tend to be client focused, rather than game focused.
But there’s a darker side to MPQ mods - they are a security hole into which hackers and cheaters can inject their software.
Testing of mods undertaken by Existor on Team Liquid shows that any altered MPQ files are re-downloaded automatically from the Blizzard servers when the game is loaded. The official files then overwrite any altered resources. This is same system is that is used by Blizzard to prevent any alteration of Diablo 3’s game files.
The upside to this approach: every copy of StarCraft II installed worldwide will be identical.
The downside to this approach: every copy of StarCraft II installed worldwide will be identical.