Make watermark size configurable

css/_base.scss

     display: block;
     position: absolute;
     top: 15;
-    width: 186px;
-    height: 74px;
+    width: $watermarkWidth;
+    height: $watermarkHeight;
     background-size: contain;
     background-repeat: no-repeat;
     z-index: $zindex2;

css/_variables.scss

 $unsupportedBrowserTitleFontSize: 24px;
 $unsupportedDesktopBrowserTextColor: rgba(255, 255, 255, 0.7);
 $unsupportedDesktopBrowserTextFontSize: 21px;
+
+/**
+ * The size of the default watermark.
+ */
+$watermarkWidth: 186px;
+$watermarkHeight: 74px;