kraweb First --> it should change the font color of the top menu (white to gray)
Try add this to custom.css file and change color code accordingly:
.style-dark .navbar-default .navbar-nav > li > a{color:#ccc !important;}
Second --> it should change the font color of the intro text (gray to white)
The CSS suggested by Pankaj is appending fine and derives the result as envisaged. Ref: Screenshot
Kindly ensure to clear your browser cache if changes dont reflect.
Note : At times, !important directive added will ensure that the specific CSS rule-set/declaration will apply so you may require to add the same when required