The default core theme comes preinstalled and ready to go with every new project. Learn how to use some of the many helpful utility classes that are provided like common btn classes for Buttons, h1, h2, etc. Label classes and convenient p-10 or m-10 (shorthand margin/padding) classes ranging from 0 - 30. 

Check out the page: http://docs.nativescript.org/ui/theme

[NativeScript] Style NativeScript views using the default core theme