Messenger Plus! for Skype is not allowed to modify the look of Skype in any way, shape, or form. Therefore you will not see themes for Skype unless Skype themselves change their Terms of Use for their addons and allow modification of their interface.
Please refer to this thread:
Why are you violating the Skype API Terms of Use?. Please note that it was determined that Messenger Plus! was not in violation of the Terms of Use. The user complaining is also a big member of the Skype community however not affiliated with the company whatsoever. Messenger Plus! for Skype is listed on the official "Skype Addon Store" or whatever they call it.
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