Visit Admin > Appearance > Settings > Bookpoints Customizable
Scroll down to the "Extensions" section and make sure the "Font" tab is selected as in the following screenshot.
You can change the default font, or expand the sub-sections to change the font for Titles (site title, page titles, etc.), Headings (such as blocks and widget titles), and Menus (such as the Main Menu).
By default, select "Websafe fonts," which are organized into roughly-similar "font families."
If your library uses Google Webfonts, you can select the "Basic Google font" option and enter the name -- exactly -- of your preferred Google font.
The "Size" field for any font is given in "em" units. 1 em = the size of "normal" text on the page. So if the normal size on the page is 14-point, and you set the Site Title to 3 em, it will appear in approximately 42-point font. You may need to tweak the size a few times until you're happy with it.
Remember to check all changes on phones and tablets!