Cleanup default styles

:koala: :bug: 

This PR cleans up default styles to match [ZAM](https://github.com/zendesk/zendesk_app_market/blob/master/lib/zendesk/app_market/app_package/default_styles.scss). This was causing styling inconsistencies between apps running with ZAT and uploaded apps. Eventually we should change ZAM to use these default assets from ZAS, so we don't have to maintain it in two different places.

/cc @zendesk/quokka