combining theory with practice
It is impossible for a leader to understand all of the leadership theories. In fact, many leaders lead their followers without reading any leadership theory. This doesn't mean they that don't understand leadership, they learn it through practice and experience instead of reading books and articles. Each leader has their own leadership style but some of them build their style without noticing they are doing so. They also don't know what their style is. But this doesn't affect the effectiveness of their leadership. Some of them even don't want to learn some related theories from books because they point out it will affect they opinions. I don't want to challenge the useful of books but there are lots of ways to learn knowledge without books. For example, some people prefer to learn through their real experience while some people obtain knowledge from books. It's hard to illustrate which kind of leader is more effective. Or perhaps it's better to combine theories with pratical experience.