Pico CMS customize template

I got PicoCMS installed and created a very primitive page with the default theme.
The markdown files with the content are in a local folder in my nextcloud.
At the moment there are these two folders in my nextcloud:

assets
content

So far so good.
But I’d like to customize the theme as well.
If possible, i don’t want to mess around with the theme files on the server itself.
So I would be interested, if I can customize the theme locally in my Nextcloud files.
Is it possible to add the necessary theme files locally, e.g.

assets
content
themes
config
...

If this is possible, what do I have to do?
Which files do I need locally and which do I have to adapt?
Thanks for all hints…

@Franky1 - I have exactly the same question. Did you ever discover the answer?

Thanks!

This is not possible due to security reasons. You can add (and modify) custom themes via FTP though.

As a workaround you could use a local external storage to publish the folders in your Nextcloud. However, you MUST make sure to share it with users having FTP access anyway only. This is neither recommended nor supported!