Apple published a book called the Human Interface Design Guidelines. This covers everything about how to design computer interfaces. I had a print copy some years ago. You can now read it online. It covers everything from color to working with text in different languages.
Apple has also written an IOS version. Here is a great quote from it.
Deference. The user interface helps users understand and interact with the content, but never competes with it.
Clarity. Text is legible at every size, icons are precise and lucid, adornments are subtle and appropriate, and a sharpened focus on functionality motivates the design.
Depth. Visual layers and realistic motion impart vitality and heighten users’ delight and understanding.