Custom subtheme w3-theme-custom setting point to contrib theme folder and not to the custom theme folder

When trying to use this setting:

BTW, w3-theme-custom is a file for your custom theme colors you can generate from https://www.w3schools.com/w3css/w3css_color_themes.asp

on a custom subtheme, and copying the css file css/w3-css-theme-custom/w3-theme-custom.css to the custom subtheme css folder, you expect this file was read by your theme, but that's not happen, but the file readed is the module file /themes/contrib/d8w3css/css/w3-css-theme-custom/w3-theme-custom.css

What's the problem? Next time this contrib module will be upgraded with a new version, custom modifications of this file will be overriden.

So, the custom subtheme should point his own file, if exists. Probably it could be created on the subtheme template.

I could try to create a patch to do it, if required ;-)

Regards