theme: drop .views CSS class

The default class for content viewers in Gtk+ is .view and
ideally provided by the theme. Not only that, but it provides
the exact same functionality of ours, but in a way that is
overridable for theme designers.

Fix this issue by removing the class from out CSS.

https://bugzilla.gnome.org/show_bug.cgi?id=765664

data/theme/gtk-styles.css

@@ -1,11 +1,5 @@
 @define-color event_shadow_color alpha(black, 0.2);
 
-.views {
-    /* background color of every view */
-    background-color: @theme_base_color;
-    background-image: none;
-}
-
 .nav-button {
     background-color: transparent;
     background-image: none;

data/ui/window.ui

@@ -50,7 +50,7 @@
                 <property name="transition_type">crossfade</property>
                 <signal name="notify::visible-child" handler="view_changed" object="GcalWindow" swapped="no"/>
                 <style>
-                  <class name="views"/>
+                  <class name="view"/>
                 </style>
                 <child>
                   <object class="GcalMonthView" id="month_view">
@@ -276,9 +276,6 @@
         </child>
       </object>
     </child>
-    <style>
-      <class name="views"/>
-    </style>
   </template>
   <object class="GcalSearchView" id="search_view">
     <property name="visible" bind-source="search_bar" bind-property="search-mode-enabled" bind-flags="default | sync-create" />