demo.css: fix demo options not shown on Chrome

Chrome doesn't allow changing the background color of the <option>
but it allows changing the text color.
As a result, the text was white on a white background.

Since Firefox allows background styling, the <option>
background is standardized to white so it works the
same in Firefox.

src/styles/demo.css

@@ -270,6 +270,11 @@ body .dg .dg.main.a {
   -webkit-appearance: none;
 }
 
+.matter-select option {
+  background: white;
+  color: black;
+}
+
 .prevent-zoom-ios .matter-select {
   font-size: 16px;
 }
@@ -412,4 +417,4 @@ body .dg {
   .matter-btn-source {
     display: block;
   }
-}
+}