DB AppMaker uses Ionic Framework. Ionic is built on top of Sass, which makes it easy for you to change Ionic's defaults. Sass is like CSS, but with extra features such as variables, mixins, and for loops.
Sass Variables allow you to define a value once and use it in multiple places. Variables begin with dollar signs and are set like CSS properties. Sass variables go hand in hand with Ionic. In Ionic, you can modify the Sass directly, so that the CSS file that gets generated has the customization you want.
You can override Ionic's Sass variables in order to get a custom style for your app, read Overriding Ionic Sass Variables first.
Then you can add your variables in the Theme tab.
The Styles tab shows the additional SCSS (other than Ionic's) to be generated by the project. There are 2 sub-tabs, App and User.
The App tab show the SCSS to be generated. It is for preview only and is not editable.
The User tab show the user SCSS for the project. You can enter any custom SCSS to change the look and feel of your app. You can even copy SCSS from the App tab to override it.