Are you asking about changing the design of the UI components in your plugin?
Or are you talking about the UI on the artboard? If so in the
File there is a new menu item
Get UI Kits > that links to UI Kits you can download. These use vectors so when you resize or export they will stay sharp.
If you are talking about the UI components in the plugin I think there are two themes you can use right now. The default and a
quite theme. If you want the quiet theme then when you create your UI component set
"myInput.uxpQuiet = true;".
On the examples page on Github search for uxpQuiet and you will see examples changing the theme.
If you use your own images in the plugin it shouldn’t be blurry. Make sure the width and height of your image in the plugin match the actual width and height of your image and that it’s not positioned on a half pixel.