Perhaps you have noticed the field named "Custom Code" in site administration. What does it do? As name suggests you can inject any HTML code to your website. Note that all the code injected is not escaped and inserted as is. Beware of any malicious code and use only scripts you trust.

Example

<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=IBM+Plex+Mono&family=Playfair+Display:ital,wght@0,400;0,700;1,400;1,700&display=swap" rel="stylesheet">
<style>
:root {
  --default-font-family: 'Playfair Display', sans-serif;
  --code-font-family: 'IBM Plex Mono', monospace;
}
</style>

This example code will change main font to 'Playfair Display' and monospace font used for code previews to 'IBM Plex Mono', both loaded from Google Fonts library.