$secondary are the two main branding colors used in the example app. They are applied to buttons, drop-down menus, icons and several other elements. By changing the value of these two main variables you can easily make the example app your own. The other variables starting with
$theme define colors which are mainly used for section backgrounds.
In total, eight sets of those colors exist in the Example App, representing themes you can choose from. To change the theme to be used, comment out the active one and remove the comment marks from the one you want.