How to download my custom theme?

I’m taking over a website that was previously managed by another person, which had installed a self-made custom theme (uploaded through .zip file). In other words, the theme is not available online. It was made for the website.

The person does not have the original files and I need to update it. Is it possible to reconstruct/download the theme so I can modify it offline and then re-upload it? I do not find anywhere a download option for the theme.

Alternatively, is it possible to bulk download the website and somehow reconstruct the theme? In principle, all should be there.

I know there are some related posts here but can’t find the proper answer to my question.