Here’s a library with three Custom HTML prompts with some styling and JS to make them feel Mac-like. There are three in here - one for a simple alert, one for selecting multiple checkboxes from a “pick list,” and another to select an item from a list with auto-complete (like a combo-box).
You’ll need to put the html-elements folder in ~/Library/Application Support/Keyboard Maestro/ so that the dialogs can access them. If you’d like to put them somewhere else, just update the variable in macros accordingly.
[Download from blog.nik.me]
####UPDATE: 2016-06-21 -- See updated macro below: https://forum.keyboardmaestro.com/t/fancy-mac-like-html-prompts-library/3361/17?u=jmichaeltx : http://blog.nik.me/post/142530887153/fancy-keyboard-maestro-custom-html-prompts