If I press a button for, for example, a highlighter, the function is selected and then after using it, the tool is switched off again. This behavior is unnatural, I don't put a highlighter down every time I work on a document. My wish: If you press the button to select a highlighter longer, it should be selected and only go out when you click again.
Read the manual, it will tell you how to do that. Or update to a newer okular and it will do that by default.