Game Room

This achievement is now available for free, without the need to buy any games.

You need to apply 12 different themes to your arcade to earn this achievement, and all of them are bundled with the initial game and Game Packs 1, 3, 4, 5, 7, 9 and 10.

The generic "[color] Room" themes do not count towards this achievement.

Free Themes:

Activision
Arctic
Atari
Caveman
Electro '80s
Fantasy
Graveyard
Intellivision
Konami
Lost Temple
Pirate
Wild West

Unlockable Themes:

Haunted House (level 2)
Heavy Metal (level 12)
Groovy '60s (level 22)
Military (level 30)
Sci-Fi (level 40)

Following the release of Game Pack 10, which contains the free 'Caveman' theme on Wednesday 22 September, this achievement can now be attained without purchasing any Game Room titles.

Simply download packs 1, 3, 4, 5, 7, 9 and 10 and work your way through your arcade, adding a different free theme to each room. Note that the default rooms- those named "The (Colour) Room" do not count towards this achievement.

This achievement is now available at any level just download all game packs and change different themes in all 12 rooms. You can of course also use theme's gained by leveling up. The last 4 color generic themes don't count.

Note: Game Pack 6 and 8 has no theme.

This achievement requires you to have a different them in all 12 rooms of your arcade. However the four generic colors that came already in the rooms do not count. Obtaining more themes requires the user to download free DLC named "Game Pack". Microsoft launched Game Room on March 24th, 2010 along with Game Pack 001 & 002, containing very few themes and making this achievement unotainable. Microsoft said they would eventually release enough free themes in game packs to unlock this achievement for free.
As of:
May 19th, 2010- with Game Pack 004, this achievement became unlockable at level 22.

August 25th, 2010- with Game Pack 009, this achievement became unlockable at level 2.

September 22nd, 2010- with Game Pack 010, this achievement became available at any level, without the need of any purchase.

October 27th, 2010- there are currently 011 Game Packs released, out of which all include themes except for 006 & 008. In total there are 12 free themes and 5 unlockable themes.

Free:

Activision
Arctic
Atari
Caveman
Electro '80s
Fantasy
Graveyard
Intellivision
Konami
Lost Temple
Pirate
Wild West

Unlockable:

Haunted House (level 2)
Heavy Metal (level 12)
Groovy '60s (level 22)
Military (level 30)
Sci-Fi (level 40)

As like the Achievement "Theme Filler" this Theme has to be done so that none of the Themes are the same, just descovered it when I had to change a few Themes around.

To Add: downloading the Game Packs mentioned above (1, 3, 4, 5, 7, 9 and 10), there is enough Themes to cover approximately 1.5 to 2 Arcades (that is without the ones Unlocked at Levels and the Four at the End, which are unusable)