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;