v4 - Navbar bg-primary dropdowns should get black font color

[TheZoker]

When a navbar has .bg-primary the dropdown should get a black font color.

Something like:

.bg-primary .dropdown-menu {
    color: #000;
}

Otherwise this leads to some problems:

[kkirsche]

Took the idea here which I think is correct and figured using the user's default body text color would probably make more sense in #17277 — thanks for reporting the bug!

[cvrebert]

Believed to be addressed by #17277.