Harmony is the coherent relationships of all of the elements in the design. Everything works together well. This way, software can be more than the sum of its parts.
This must involve good typography, layout, and colour choices.
The style used to decide how elements look also needs to be considered. Especially illustrative elements like symbols and illustrations. They should feel like they belong together.
The design should also be suitably dense. This doesn’t necessarily mean that the software will be dense or spacious, but whatever is appropriate for the people who use it.