Styling add-ons with Atlassian UI
Because style properties from the parent page don't cascade down to your add-on's iframe, you'll need to control your page's design to match the parent. Using the AUI will help you with this task.
Using AUI styling in your pages
You can include the AUI resources on any page that you serve from your add-on. In most cases, you should use the most recent version of AUI that is available. It's your responsibility to update as new versions become available. Not only will you get new features and bug fixes, but using the most recent version of AUI ensures that your styles will most closely match those of your host application.
To include AUI resources in your add-on, please see the AUI documentation.