Design theory is all about making things look good and work well. Imagine you are drawing a picture. You want it to look nice and be easy to understand. That's what design theory tries to help you do.
One important thing about design theory is that it looks at how things work together. Just like how you need all the pieces of a puzzle to fit together, design theory looks at how all the parts of a picture or object fit together to make it look good and work well.
Another important thing about design theory is that it wants to make things easy to use. If something is hard to use, people won't want to use it. That's why design theory tries to make things simple and easy to understand.
Design theory is used in many different things, like buildings, clothes, technology, and even food packaging. It helps make things look nice and work well so that they're useful for people.