Some of the graphics provided with VuFind®'s themes (presently or in the past) came from the FontAwesome, Fugue, Silk and Tango icon libraries. For more information on licensing terms and to find additional icons for use in your local customizations, see these links:
It is sometimes useful to add more icons for local customizations. This page provides some possible sources for additional icons.
Assuming compatible licensing, it may be desirable to adopt an existing icon set and fill in missing images by applying the same general style. Here are links to some existing sets that may be worth examining: