On a computer
To set a preferred font for all your new documents or presentations:
- Open a file and begin typing.
- Highlight the text and, from the toolbar, select the font you want to add as the default.
- Click the Format menu.
- Hover over "Paragraph styles," and then "Normal Text," and select Update 'Normal text' to match.
- With the text still highlighted, click the Format menu again, hover over "Paragraph styles," and then "Options," and select Save as my default styles.
Once the default font has been set, all new documents or presentations that you create will use this font until you change the default option again.
Note: At the moment, this feature isn't available for spreadsheets.
On an iPad
Change your font
- Open a file in the Google Docs, Sheets, or Slides app.
- Double tap a word, or cell in a spreadsheet, to select it. Move the blue markers to expand the text that's selected.
- At the top of the screen, tap Format TEXT Font.
- Choose a new font from the list. Your text will change when you choose an option. Tap back on the document to continue editing.
Note: You cannot set a default font for your documents or add fonts to your font list on mobile.
Source: Google docs editors help page