Set Default Styles in Google Docs
In a blank Google Doc, Type Normal Text.
Format the text to look the way you want.
Highlight and go to the style menu, hover over normal text, and in the menu choose Update Normal Text to match.
Now follow the same steps for Title, Subtitle Heading 1, and Heading 2.
Finally go to the style menu, then options and choose, Save as my default styles.
All new documents will already have your customized styles ready to go.