Fixed _drawing.scss dark shadows only.

White shadows produce some weird looking results in some cases, where not only text but also icons look broken. Also dark shadows seem to work fine with both normal Adwaita as well as Adwaita-dark, I also tested some lighter themes and they also look fine with dark shadows and the icons look even more broken there with white shadows. So I think the white shadows just bloat the code and produce ugly results and therefore should be removed.

Before:

With Dark shadows only: