Add system theme colors for FloatingButton (#112)

resources/styles/system.qss

@@ -90,6 +90,11 @@ ScrollBar {
     qproperty-backgroundColor: palette(window);
 }
 
+FloatingButton {
+    qproperty-backgroundColor: palette(window);
+    qproperty-foregroundColor: palette(text);
+}
+
 QListWidget {
     background-color: palette(window);
     color: palette(text);