Excellent question, and one that they've decided to dodge despite it being asked countless times since the feature was dropped in MX4.
Part of the problem is that I don't think it's technically an officially supported feature, even though there is documentation explaining the details (albeit much is from MX3/4). So users that know create and modify the skins, and offer them to others.
On top of that the skin.maxthon.com page still shows " Maxthon Cloud Browser skin feature will see you soon. Please stay tuned.:) For more Maxthon news, please visit ". Still no info on the main page that the skins on the site don't work with MX4/5. Given that MX3 is long since deprecated, there's no reason to even keep the site online anymore.
Thankfully Wilser still does create some skins for users despite no support from MX to make it easier for users to add them.