Just fyi... I have been successful in leveraging HTML service with GAS. I create a standalone script, code up a webpage (using all the css I want) and connect the script to a spreadsheet. Here is a link to a set of videos where they show how easily this can be done:
https://www.youtube.com/watch?v=RRQvySxaCW0&list=PLv9Pf9aNgemt82hBENyneRyHnD-zORB3l Part 1 does a great job of doing a very quick example of the process.
By the way, a published stand alone script will have a "gray info/warning" bar at the top of the page which is a little annoying. To get around this you can easily embed your app in a Google Site.