Todo app using react & typescript

This is my first React project. In this mini-project, I encountered many problems that I saw for the first time, so I focused on solving them, but I did not solve this challenge cleanly and correctly, for example, switching themes and styling elements. I put some styles in the main css file, I used some styles from the css module, and I also put the whole mini-project in one component file, which I know was not done correctly, and I just solved the challenge, which of course has some flaws. I am in the process of learning this front-end path. What suggestions do you have for learning or even writing the code correctly and my problems? Thank you