fix(grid-list): darken grid footer (#1310)

Author: kara

src/lib/grid-list/grid-list.scss

@@ -49,7 +49,7 @@ md-grid-tile {
     align-items: center;
     height: $md-grid-list-one-line-height;
     color: #fff;
-    background: rgba(0, 0, 0, 0.18);
+    background: rgba(0, 0, 0, 0.38);
     overflow: hidden;
     padding: 0 $md-grid-list-text-padding;
     font-size: $md-grid-list-font-size;