added some forward and backward buttons to gallery

vue-theme/src/components/gallery.vue

 <template lang="pug">
-.gallery(v-if="fullscreengallery")
+.gallery.f-col.center(v-if="fullscreengallery")
+    button(@click="hideGallery") hide
     ul
         li
             h1 hello world
         li
             h1 hello world again
-    button(@click="hideGallery") hide
+    .controls.f-row
+        button(@click="hideGallery") <<<
+        button(@click="hideGallery") >>>
 </template>
 
 <script>
         z-index: 1011
     ul
         list-style: none
-        li
-            h1
-                color: green
+        h1
+            color: green
 </style>