In the settings tab fonts, you can configure the fonts displayed in the designer.
System fonts are common fonts, find a list of common fonts at this address: https://web.mit.edu/jmorzins/www/fonts.html.
Fonts files must be placed in the ask_fonts folder of the uploads folder of your WordPress installation, if the folder does not exist, create it. Files must be in ttf, woff or otf formats.
The google fonts should not be too large, to avoid too long loading time.