I discovered that I can use 3.x themes from https://www.npmjs.com/package/swagger-ui-themes by adding the css file to Assets and then add this in the Custom Code section in settings, which gets added to the bottom of every page:
You can control some behavior by looking at the swagger-ui.css. For example if you wanted to turn of the “Models” section at the bottom of the page, you could add the following to Custom Code: