Upgraded all themes to Azurra framework

What is this Azurra framework you might ask? It's our in-house development pack to easily create and maintain themes using SCSS. Now everything's ported, but there's a lot of optimisations to do, so we'll attack thos next.

We also added a couple of themes:
- Windows Phone 8.1 Dark
- Windows 10 Acrylic (previously Mobile)
- Windows 7 (previously Se7en, now has a GTK theme)