Fix dark text on dark background in combobox dropdowns on windows (#3315)

Author: UdjinM6

src/qt/res/css/dark.css

@@ -185,6 +185,7 @@ border-image: url(':/images/arrow_down') 0 0 0 0 stretch stretch;
 }
 
 QComboBox QListView {
+color: #aaa;
 background: #555;
 padding: 3px;
 }