When you aren't a designer it can be difficult to create a good looking interface for your users. And despite what you think, it matters to your users!
One tool that lots of developers use is Codepen. It allows you to get real-time updates on your code AS YOU CODE! How cool is that?!?!
It would be incredible if the Google Script Editor had the same functionality, however, we will work with what we have. And until the App Maker is open to the public we must use tools like Codepen. Fortunately, Codepen is incredible!
Not only does it provide the tools needed to layout your frontend code, but it is a code repository of work from incredible designers. If you struggle with design, there are a plethora of ideas that you can browse and get that spark to get you going.