Inheriting CSS in child theme’s child theme

I’m relatively new to this, so i hope someone can help me out.

In my store i have a child theme of blank. The changes in this child theme get properly displayed on the store front and everything not changed is inherited from blank (as it should be).

Now i need another child theme of this child theme to make some additional changes for specific sites in _theme.less. So i tried to create a child theme that inherits from my first child theme that inherits from blank. But the result is that it does not inherit the changes i made in _theme.less in the first child theme. New changes in _theme.less work but everything else is inherited from blank instead of the child theme of blank.

If i delete _theme.less in the child-child-theme it displays everything as changed in the first child theme. So i think the problem is that the new _theme.less overrides the _theme.less from it’s parent completely. Shouldn’t the child-child theme inherit everything from the child theme’s _theme.less except when it is changed in it’s own _theme.less?

Would be great if someone could show me a way out of this problem.