Please adjust documentation regarding custom.css path.
In your documentation (https://www.joomlart.com/documentation/t4-framework/css-scss-customization) you have the information for custom.css.
T4 Framework allows user to add their custom CSS in the custom.css file inside template folder:
templates/t4_blank/css/custom.css
By default, the file is not included in the template folder so the user needs to create the file by themselves. The file is not overridden when upgrading template to new version.
I tried it and found out that it only works there:
templates/t4_blank/local/css/custom.css
best regards,
Ayko