Alternatively, if you decide to change your mind about child theming and want to just keep things simple, you can simply paste the following CSS into your theme options (https://startupwp.com/topic/customizing-your-theme/):
@import url(http://fonts.googleapis.com/css?family=Bree+Serif);
h1, h2, h3, h4, h5, h6, h1 a, h2 a, h3 a, h4 a, h5 a, h6 a{font-family:'Bree Serif',serif}
body, div, p{font-family:arial,sans-serif}