css: Add styling for checkerboard

data/image-viewer.css

@@ -12,3 +12,29 @@ headerbar.osd {
 .image-view {
   background-color: black;
 }
+
+.checkerboard picture {
+  background-color: #7f7f7f;
+  background-image: linear-gradient(
+	  45deg,
+	  #cccccc 25%,
+	  transparent 25%
+  ),
+  linear-gradient(
+	  -45deg,
+	  #cccccc 25%,
+	  transparent 25%
+  ),
+  linear-gradient(
+	  45deg,
+	  transparent 75%,
+	  #cccccc 75%
+  ),
+  linear-gradient(
+  	-45deg,
+	  transparent 75%,
+	  #cccccc 75%
+  );
+  background-size: 20px 20px;
+  background-position: 0 0, 0 10px, 10px -10px, -10px 0px;
+}