How can I change z-index of tooltips? (to bypass a Valo issue).

I am facing an issue with Valo theme in menu tooltips that get covered by drop-down menu (see image). It is probably an issu of Z-INDEX and I already reported the issue in the
sub-fourm, but before the issue gets solved, I would like to apply a temporary fix to my CSS.

Do anybody knows where is the z-index of tooltip defined? I was unable to find it. Thanks for any kind of help.