Hello, that's a good idea for people who use the sidebar with a single site.

I've just added this option in version 2.1.0.
Remember to update the extension.

When you only have one favorite (like Proton Mail), an option appears in the settings: "Use the name and icon of … instead of Universal Sidebar". Once checked, the site's name and favicon replace those of Universal Sidebar, so "Proton Mail" with its icon, exactly as you wanted.

If it doesn’t work as expected, you can email me at contact@mrweb.fr.