Scroll down to ‘Theme’ in the App Browser, and click on ‘Assets’:

Location of 'Assets' in the App Browser

In the dropdown at the top, select theme.css, and add this line to the top of your css:

/**
 This CSS implements the Material Design look and feel for Anvil apps.
 **/
@import url('https://fonts.googleapis.com/css?family=Open+Sans&display=swap');

Scroll down to the ‘Typography’ section of theme.css (around line 83), and modify the css that styles the body of your app to look like this:

/* Typography */
body {
  font-family: 'Open Sans', sans-serif; /* edit this line */
  font-size: 14px;
  line-height: 1.4286;
  background-color: #fafafa;
}

Run your app, and you’ll see the font has now changed!