Where is the Customizr style switcher ?

The style switcher is located in the live customizer > Advanced options panel.

Since Customizr 4.0 and Customizr-Pro 2.0 you have the ability to choose a design style for your theme: Classical or Modern  (default).

While the  Classical style is mostly based on a "flat design" approach with support for both modern and older devices and browsers, the Modern style is more inspired by a "material design" and it's implemented on top of one of the most recent web design technologies: the CSS flexbox.