Fix version z-index and button color

Author: andrewjong

docs/stylesheets/extra.css

@@ -14,6 +14,10 @@
     z-index: 10;
 }
 
+.md-version {
+    z-index: 10;
+}
+
 /* Splash container and background */
 .splash-container {
     position: relative;
@@ -99,6 +103,7 @@
 }
 
 .mdx-hero .md-button {
+    background-color: #ffffff;
     margin: 0.5rem;
     font-size: 1rem;
     padding: 0.625em 2em;