Custom HTML in an Atlassian SDK Confluence Plugin

Hi folks! I’m building a plugin for Confluence Data Centre to apply version control and enable a deployment pipeline for some customisations we have in a User Macro. That part is relatively straightforward. We also have a bit of Custom HTML (under ‘Custom HTML’ in General Configuration) that adds a few elements, some CSS, and imports and runs a few scripts. Are there any examples of plugins that do this?

To be clear, this is not a theme, merely a few tweaks to the default HTML.

Apologies if this is well-covered in the docs; I couldn’t find exactly what I am hoping to do.