Unable to copy new style file into Notepad++ under /snap

I installed Notepad++ in Ubuntu 18.04.5 LTS using Ubuntu Software. Notepad++ was installed under /snap.

I am trying to copy a new style file:

~/Desktop/Cobalt.xml

into the Notepad++ themes folder.

sudo cp Cobalt.xml /snap/notepad-plus-plus/238/winedata/Application\ Data/Notepad++/themes

I get the following error:

cp: cannot create regular file '/snap/notepad-plus-plus/238/winedata/Application Data/Notepad++/themes/Cobalt.xml': Read-only file system

I was able to copy the file in folders /snap and /snap/notepad-plus-plus, but somehow from folder /snap/notepad-plus-plus/238 it doesn't work anymore...

Any idea how to go around this ?