I agree with @PPetree’s answer.
For commercial grade projects, I use a professional designer. One of the guys on the team here (Tomo) does great designs: colors, icons, building the UI for the app. He’s a professional architect, as well as a graphic designer and software developer.
For the rest of us, without his abilities and qualifications, it’s not as easy. Certainly @PPetree’s suggestion of looking a good designs and following them helps.
Bootstrap and jqWidgets help by having nicely designed and consistent controls.
You’ll want to get used to Responsive Web Design. There are also several articles in our Wiki which may help.
AppStudio has two special controls which really help with organizing a responsive layout:
The CSSGrid control provides a grid-based layout system, with rows and columns, making it easier to design web pages without having to use floats and positioning.
The Flexbox control makes it much easier to design a flexible responsive layout structure without using float or positioning.
You’ll find your apps will make much more use of nested controls - sometimes going 4 or more levels deep. Don’t let that stress you - HTML is made for that.