Coming this month: you will be able to adjust text size, color, font, spacing, and more while typing in a text box in Google Sites. This will provide customization of individual words, paragraphs, pages, and sections within your sites.
In 2021, Google will add the ability to create themes, which will allow you to easily set fonts, colors, and styles across your whole site. We’ll keep you updated when we learn that’s available.
Customize text style and appearance in Google Sites