Theme "Skins"

In a word, no.

Welcome aboard... here's some background information you might consider.
MuseScore development from 1.3 to 2.0 took over four years for a stable release.
See:
https://musescore.org/en/whats-new-musescore-2-0
and:
https://musescore.org/en/node/50996
According to those links, the emphasis was on 'new features and improvements that have been requested by users over this period.' Much of that effort went into improving the functionality of scorewriting itself (MuseScore's primary directive), and also for playback support and midi import. During that development arc, and as time 'dragged on', the forums became sprinkled here and there with the question: "When will MuseScore 2.0 be released?" Each time, the answer was the same (as it is now for ver.3.0}: "It will be released when it's released." ;-)

However, there *were* some improvements to the user interface:
https://musescore.org/en/node/50996#User-interface
but, alas, 'eye candy' such as theme 'skins' would require precious limited resources to implement/maintain. Just have a look at the discussions regarding the current icon set for MuseScore 2.0:
https://musescore.org/en/node/21048
https://musescore.org/en/node/21051
Imagine the extra time and input that would be needed for a consensus on the designs (much less color choices) for 'skins'!

Also, MuseScore is cross platform; and, being a technical application, often requires user support via these forums. Uniformity makes it so much easier for a forum contributor - e.g. who posts something like a screenshot (for a question or an answer) - to clarify a concept, knowing that the other person 'sees' the same image on his MuseScore screen.

That's probably why you see the smiley after Marc's reply.... :-)

Regards.