To customize the theme with your own fonts, please install and active the Easy Google Fonts plugin.
After you have done so, you can start adding font controls from Settings › Google Fonts. The font controls can be associated with css classes that we used in the theme. For example, if you wish to change the default theme font for several html tags, such as headings, blockquotes and all the small texts with another font, use the html tags from the following screenshot:
After adding the font controls with the associated classes, you can choose the font families from Appearance › Customize › Typography › Theme Typography.