How to know wich variable I need to change for themming apps

Hi! I’m new to Ionic framework and I want to start themming my app, but I couldn’t understand how am I suppose to know wich variables I should override in variables.scss to achieve things like: change ion-navbar background, for example, or text color, and so on.

The cool thing about Ionic is that you can make your own styling with different ways. Basically you will need to use sass or css to themming your app. I advice you to start reading some tutorials, like this one A Guide to Styling an Ionic 2 Application, from Josh Morony.
Hope it helps you.