How does an app get or find toolbar icons?

On Manjaro Linux, I installed the Okular PDF viewer. On its toolbars are various icons (e.g. the "hand" icon used for scrolling with the mouse). My questions are: - Are these icons part of the Okular installation, or are they already present on my system? And if they are already present, how does Okular know where to find them? - If I want to modify the appearance of these icons (for Okular only), what options do I have and where can I find them?