T3-bs3-blank, custom.css in theme not used ?

Hi,

I am completely new to T3 and I have been trying to alter the background color of the megamenu dropdown. I accomplished this by putting this code into
\templates\t3_bs3_blank\css\custom.css

.mega-dropdown-menu {
background-color: #669999;
}

The issue is, It seems that custom.css only works in the "main" directory (templates\t3_bs3_blank\css) and not in the "theme" directories e.g. "templates\t3_bs3_blank\css\themes\red" . is that normal behavior / as designed ?
(of course I set T3 to use the "red" theme).

Tnx, Carel.