termsofuse.html "Game clients and Service. The Game clients and the Service (including without limitation any titles, computer code, themes,

See 9.a. here: us.blizzard.com/en-us/company/about/termsofuse.html
"Game clients and Service.
The Game clients and the Service (including without limitation any titles, computer code, themes, objects, characters, character names, stories, dialogue, catch phrases, concepts, artwork, animations, sounds, musical compositions, audio-visual effects, methods of operation, moral rights, documentation, in-game chat transcripts, character profile information, recordings or replays of Games, and the Game client and server software) are copyrighted works owned by Blizzard and its licensors. Blizzard reserves all rights in connection with the Games and the Service, including without limitation the exclusive right to create derivative works. You agree that you will not create any work based on the Games or the Service except as expressly set forth by Blizzard in contest rules, or in Blizzard's Fan Policies, which include without limitation Blizzard's Machinima Policy and Fan Art Submission Policy. Any reproduction or redistribution of any Game not in accordance with any policy or agreement, including without limitation the Game EULA and the TOU, is expressly prohibited by law, and may result in severe civil and criminal penalties."

Also

"Can I write novels, screenplays, theatrical productions or other adaptations based on your games?
No. Blizzard Entertainment® reserves the right to extend and expand our properties to other media. We want to provide a consistent story and universe for our customers, and want to ensure that only the highest quality, officially licensed and approved material is created based on our characters and other creative properties."

BUT

What is Blizzard Entertainment's® copyright/trademark policy for the Internet, specifically for fansites? Can I use Blizzard Entertainment's® images, text or sound on my web page? Is it ok if I use screenshots I take in-game on my web page?
Yes, within certain limits. We asked our legal department to provide some guidelines for you, and here is what they said:

Blizzard Entertainment® hereby grants you a personal, non-exclusive, non-transferable and non- assignable license to use and display, for home, noncommercial and personal use only, one copy of any material and/or software that you may download from this site, including, but not limited to, any files, codes, audio or images incorporated in or generated by the software (collectively the "Downloaded Content") provided, however, that you must include or maintain all copyright and other notices contained or associated with such Downloaded Content. You acknowledge and agree that you may not sublicense, assign or otherwise transfer this license or the Downloaded Content and that no title to the Downloaded Content has been or will be transferred to you from Blizzard Entertainment® or anyone else. You also agree that you will not alter, disassemble, decompile, reverse engineer or otherwise modify the Downloaded Content.

Also, we reserve the right to revoke this limited use license at any time, for any reason, and at the sole discretion of Blizzard Entertainment®. You may not use our materials on sites that feature defamatory pornographic, or inflammatory content, including, but not limited to, hacks and cheats for any of our games or any other content that Blizzard Entertainment® find objectionable or unlawful.

Are there any legal notices and disclaimers that I need to have on my site when talking about your products?
Yes. You must include all copyright, trademark and other notices as appropriate. Appropriate notices can be acquired from here.

Can I register a domain names containing some portion of your product names such as "star-craft.com" or "war-craft.com?"
No. We are concerned that such use could cause confusion for our customers who may assume that the domain is associated with Blizzard Entertainment®.

AND

Can I make and sell my own products (T-shirts, card games, models/figures, etc.) based on a Blizzard universe?
No. Blizzard Entertainment® does not enter into licensing agreements with individuals. To ensure the quality of all Blizzard products, all of our merchandise is created under a licensing agreement and all prospective licensees are thoroughly reviewed by Blizzard Entertainment before a license is granted. If you are a company that is interested in creating licensed merchandise and your company is looking to pursue a corporate licensing relationship with Blizzard Entertainment, please contact merchandiselicensing@blizzard.com.

so.... NO, you may create a theme for your own fan website but you can not create a theme as a product under another license that you "sell for free".