This completely drops the icon customization with icon effects and only retains the icon size settings. I also couldn't find any trace of that "animations enabled" check box it used to have.
It adds drag and drop support for installing archives as themes. It can also download them from remote location when dropped from e.g. a browser. Installing theme files actually never worked in Plasma 5 as it got broken in the port from KIO NetAccess to storedGet.
An animated preview is also added on hover showing a common selection of icons.