Fix bug where theme-popup was under the navigation arrows making it impossible to change the theme

src/theme/book.css

@@ -253,6 +253,7 @@ table thead td {
 .theme-popup {
   position: relative;
   left: 10px;
+  z-index: 1000;
   -webkit-border-radius: 4px;
   border-radius: 4px;
   font-size: 0.7em;

src/theme/stylus/theme-popup.styl

@@ -2,6 +2,8 @@
     position: relative
     left: 10px
 
+    z-index: 1000;
+    
     border-radius: 4px
     font-size: 0.7em