Icons should always have a shared visual motif so that they look and feel as if they belong together. This will allow users to understand which icons are related by the colors and styles they share. As a result, users will intuitively know which icons share similar behaviors. In addition, the chosen visual motif for a set of icons should be kept consistent at every level of the user interface. Mixing literal and abstract icon motifs, for example, can cause confusion for users when interpreting the actions and functions available on a given screen.
Visual motifs and icons in User Interface Design
Posted in 2014