Ionic has support for SASS (CSS extension language) to create, customize, and maintain CSS. It eases the process of customizing the existing colors and themes of Ionic for specific platforms. Ionic 3 as a default has SASS setup and you will find that inside the theme folder there will be an variable.scss file. In this file we will be doing custom color changes and overriding platform variables.
Earlier in Ionic 1x, the application was hooked to the Ionic's precompiled files, which you can find at the www/lib/ionic/css directory, and file resources and paths are linked in index.html. Previously, in Ionic 1x, we used to set up SASS using CLI:
$ ionic setup sass
This used to automatically remove other file paths and uncomment the ionic.app.css files used for SASS styling inside index.html. Now with Ionic 3 we don't have to set up SASS, it comes by default when we start or create an application.