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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:
The KDE software collection (of which Okular is a part) includes a set of icons for standard concepts which can be used across many different KDE apps. Or more precisely, there are several different themes, each of which provide such a set of icons with a distinctive look to that theme; the one I linked is called "Breeze" and is the default in modern KDE. Some of the icons you see in Okular come from this set, including the hand icon.
You might be able to replace these icons by creating a custom theme which has your own customized versions of only the specific icons you want, and "falling back" to the default theme for the rest of them. However, that change would be effective across all KDE apps, not just Okular. Alternatively, you could just replace the file in place, but again, that would apply to all KDE apps, and also would be reverted when you upgrade the breeze-icons package. The only way I can think of to make the replacement effective only for Okular is to run it inside a container or sandboxing tool or something similar, such as bubblewrap, which lets you replace the icon file only for that one program.
Others icons are specific to Okular and can be found in the source code. For these, again, you could replace the file in place or use a container/sandboxing tool. However, because these icons are specific to Okular, there are no themes to worry about, and the replacement will automatically only apply in Okular itself.
In general, the icons appearing in a program's interface are considered part of the program (or part of a library the program uses) and are not meant to be trivially replaceable. Of course you can change anything by modifying the program's source code and rebuilding it, but there isn't necessarily going to be a much easier way than that.
