From Add titles, headings and customize the style of your document - Docs editors Help

Customize titles, subtitles, headings and text style

With styles, you can quickly customize the look of text in a Google document. Use the new styles toolbar menu to format text in your document.

1. Highlight a selection of text with the font size and style that you'd like to use for normal text or a particular heading.
2. Go to the styles toolbar menu, and click the arrow next to Normal text or Heading 1-6.
3. Select Update Heading X to match selection.
4. All of the text in your document with the same text type, such as Heading 4 or Normal text, will be updated to match the font and font size of your initial selection.

Normal text

Normal text is the default text style of your document. If you update the style of normal text from the styles toolbar menu, the default text style of your document will change accordingly.

If you change the font style of normal text, the font style of the headings will update. If you want your headings to have a different font style than normal text, we recommend setting normal text style prior to setting heading text style.

Save default styles for your documents You can save customized styles to be used by default for new documents.

1. Expand the styles drop-down menu.
2. Point your mouse to Options.
3. Select Save as my default styles.

Then, apply that style to a new document:

1. Expand the styles drop-down menu.
2. Point your mouse to Options.
3. Select Use my default styles.

To restore your default styles to the default Google document styles:

1. Follow Steps 1 and 2 above.
2. Select Reset styles.

References
Edit and format a Google document - Docs editors Help