Custom CSS should always be added after main CSS.

Also fix issue with "undefined" IDs.

src/General/Main.coffee

@@ -90,7 +90,7 @@ Main =
     $.addClass doc, 'fourchan-x', 'seaweedchan'
     $.addClass doc, if g.VIEW is 'thread' then 'thread-view' else g.VIEW
     $.addClass doc, if chrome? then 'blink' else 'gecko'
-    $.addStyle Main.css
+    $.addStyle Main.css, 'fourchanx-css'
 
     Main.setClass()
 

src/Miscellaneous/CustomCSS.coffee

@@ -3,7 +3,8 @@ CustomCSS =
     return unless Conf['Custom CSS']
     @addStyle()
   addStyle: ->
-    @style = $.addStyle Conf['usercss']
+    $.asap (-> $.id 'fourchanx-css'), =>
+      @style = $.addStyle Conf['usercss'], 'custom-css'
   rmStyle: ->
     if @style
       $.rm @style
@@ -11,4 +12,4 @@ CustomCSS =
   update: ->
     unless @style
       @addStyle()
-    @style.textContent = Conf['usercss']
+    @style.textContent = Conf['usercss']