Answer:
The theme of the story "The White Knight"
The story teaches and makes us ponder on ‘self awareness’, whether actions are more important than appearance. The white knight by appearance that represent champion of virtue, honour, and justice by the society and himself go in search of black knight to slay it since that is what the white knight is expected to do. However, in the process, the white knight without realizing commits the deeds of black knight turning himself into black knight.
The story teaches us that appearance does not matter. It is self-awareness and good actions that makes one a true White Knight. One doesn't have to travel and be in high post to do good deeds, good actions start when we understand and change oneself and start making small positive differences in our own small ways.
The main idea of the story is not really about the knight doing harm to the forest and turning himself into being evil. The story aims to impart wisdom or self-discovery of who the White Knight really is.
While Lancelot was the White Knight, white knight has been used to name anyone who comes to the defense or rescue of someone since the early 1600s. It can also mean a “champion” more generally.