Marvin as a home-grown graphic work is defensible as 'Fan Art' but Warner owns the "Image" [character] as such so commercial use/distribution will be dependent on permission/s from Warner Bros.
The 'ideal' way to handle your wish to make use of the character [self-drawn or not] is to seek consent for its use, explaining the specifics of intended distribution [and/or sale].
Commercial use isn't necessarily impossible to achieve... it's all about appropriate approaches and consent....
Re potential issue with Stardock's Windowblinds that will ONLY be an issue for the skinner, unless Stardock were to include the 'Marvin' WB within the Windowblinds product distribution itself.
Wincustomize.com [and the other skinning sites] make a point of requiring appropriate consents be in place to absolve them [the sites] from potential issues of distribution.
Arguably there are three levels of use/distribution/consent/copyright.
1. For your own use only [NO distribution at all] - anything goes.
2. Fair use [Public distribution] - if it's your own graphics - making reference to the original as Copyrighted to whom-ever.
3. Formally consented Use - distribution [commercial or otherwise] as agreed by Copyright owner.
ANYTHING distributed for commercial gain, whether in #2 or #3 can expose the artist to legal action so the bottom line is.... if ya gonna sell it...make darn sure you ask first....