Ionic has a lot of great ready declared CSS components to make your life easier while coding your next hybrid mobile application. These components vary from a basic grid system to styling your forms. These components are in your use if you choose to install Ionic with the pre-set CSS stylesheets.
One of the basic functions that Ionic CSS brings to your hand is that it comes with a set of colors to start with, but as a general rule colors are meant to be overridden. Utility colors are added to help set a naming convention. You could call it a basic theme of the application. To customize the colors you can simply override those coming from the ionic.css CSS file. Additionally, since Ionic is built using Sass, for more power and flexibility you could also modify and extend the color variables within the _variables.scss file.
You can setup an Ionic project to use SASS very easily by running the
ionic setup sass command in your terminal.
You can find the official documentation on Ionic CSS here: http://ionicframework.com/docs/components/