Your CSS file should contain only overrides to the specific rules that you need. Don't take a copy of the default CSS and modify that, because if you do then you'll essentially be loading the same file twice, unnecessarily increasing loading time.

We've created a tool to help you figure out what selectors you need to use to override different elements.  You can use the following page and your browser's dev tools to grab the necessary selectors, and to see your CSS file in action.

The SDK styling helper: http://mockup.edentitivixverify.com/greenid-mobile/