To change a default Google Docs Header format to another size or color. Select the header you have, then change the change the front, size etc.
When you’re happy. Then make sure you still have the header selected. Then go to the header section and select e.g. Heading 3 and press the “update ‘heading 3’ to match.
Now you’re default header format for all headers number 3 will have this format in your document.