First of all, excuse my bad english... so
The site is under modifications, this is not public now.
My example :
I have 2 themes, a primary one (SKIN TO V3) , and a child theme _SKIN CAPITAINE VAUDOU (colors modifications et sommes other changes)
For the first theme, I have this files, i'ts OK for me
For the child theme, all files other than the customs.css are inherited from the main theme, only the custom.css is modified.
I made some changes in the custom.css file of the child theme, and I see when the site engine loads the CSS, that it concatenates the result by first loading the custom.css of the child theme, and then the custom.css of the parent theme.
In fact, in this case, I'm not be able du overwrite rules in my child theme.