Overriding some face attributes without changing the default(s) one(s)

[Alexandre LAURENT]

Hi,

I am trying to customize the whitespace-mode face for "long" lines.
By default, a specific face is defined for long lines but it is kind
of ugly in my case.
I would actually like these lines to look the same color as usual with
weight = bold, i.e. keeping the syntax highlighting (keywords,
strings, function names, etc.), but overriding weight or underline
attributes.

>From 
>http://www.gnu.org/software/emacs/manual/html_node/elisp/Face-Attributes.html,
it seems that setting some attributes to undefined "tells Emacs to
refer to a parent face", but not sure what the parent is in this case
anyway.

Moreover, if I don't set foreground / background at all, these are
just resets to the default face (I suppose).

Since there are multiple different faces for syntax highlighting, and
only one for long lines, I'm not sure that this is even possible. But
if it is, any ideas?

Thanks,

-- 
Alexandre LAURENT