I've actually given up on making things look native and have switched over to the Milligram framework for custom HTML dialogs. It's small and I can just fit it right in the HTML prompt macro itself. Then I can just put in form elements and they look nice and become responsive.
Here's an example with a textarea:
Custom HTML Prompt that Saves Size and Position.kmmacros (26.0 KB) - Updated on December 4th with a Mac-like "skin"