Lots of stuff has changed.
Mainly in the taskbar. Backgrounds, buttons, flyouts, jump lists etc.
Some is different in the msstyle (don't expect everything to be in the usual place). Much isn't in the msstyle at all anymore. A lot of it is XAML now.
XAML is mostly in C:\Windows\SystemApps\ShellExperienceHost_cw5n1h2txyewy in dlls like JumpView, ClockFlyoutExperience etc which I don't know how to edit yet.
There's no import button or stylehack. You'll have to port 7 and 8 msstyles manually.
That's all I am fairly sure about. Maybe somebody else can tell you more.
Thanks for the info so all theme for 10 will be incomplete because of XAML change over?