Custom HTML in themes?

I am looking to add video and audio clip upload functionality to the instance over at waveform.social since this is a music-focussed instance.

The way I am thinking about doing this is building a micro-app (similar to pictrs but with a mini frontend) that allows my users to upload audio and video clips and adding the required metadata to the pages to allow embedding.

I would love to add a few buttons (in nav and on the create-post) page to link to this micro app to allow users to find it more easily. My first thought was to create a custom theme. But from reading the docs, I can only see mentions of css.

Is there a way for me to modify the html to add these buttons? Or do I have to fork lemmy-ui to be able to achieve this?