Customize the Appearance of the Widget
To match the look of your site, you can add custom CSS to be included along with the page. To specified CSS file URL go to your Developer Portal, where you should see a section called "Web Dialog" an set "URL for Dialog v2 CSS" field. You can also specify css file url per dialog instance in picker options. The URL you use to link to your CSS file must use https:// or it will be ignored. Since dialog v2 was completely rebuilt from the ground up, custom css files from v1 will not work correctly with it.
You can find an un-minifcated version of the CSS file used for Dialog v2 here. Please feel free to experiment and make the Filestack your own.
Custom CSS examples