Classic Shell is a must on any post-7 Windows, but that in itself won't vanquish the flat interface. For that, you will need a new Windows theme.
The UWP-themed bits of the OS are another question. The last time I personally investigated theming 10, they could not be themed by conventional means (.msstyles editing). In 8.1, I dealt with apps by sending them straight into the fiery depths of Hell, but you can't do that with 10, now that they're phasing out the control panel in favor of the settings app.
Windows 10 can join my Win 8.1 apps...
Biting the hand that feeds IT
