Issue #1793 - Add a border-bottom to toolbox

Follow-up to e8be661944c730cb3b702cd7f7d97e0860eb9132

toolkit/themes/windows/global/toolbar.css

@@ -23,6 +23,12 @@ toolbox {
        so don't use the widget toolbox styling and fallback to -moz-Dialog's
        color which by default should be a non-blinding rgb(240, 240, 240) */
     -moz-appearance: none;
+
+    /* Since we are overriding the style we want to provide the bottom border
+       that is normally drawn by look&feel.
+       However, local application styling may also provide it on adjacent
+       widgets as a border-top. Those will need to omit it on Windows 10. */
+    border-bottom: 1px solid ThreeDShadow;
   }
 }