There are a ton of customisation options in the accessibility settings in iOS. When doing things the Apple way, this options should, in most cases, just work for you. If not, you can find flags to check for all these options in UIAccessibility.

Display & settings section, inside the Accessibility settings in iOS. It shows several options that can be turned on such as: bold text, button shapes, on/off labels, reduce transparency, increase contrast, differentiate without color, smart invert or classic invert.