Resample background colors whenever stylesheets change

This prevents a flicker that occurred on theme changes when we just
detected this via polling. I'm going to leave the polling as a catch-all
in case the editor is styled inline in some way.

src/editor-component.coffee

@@ -679,6 +679,7 @@ EditorComponent = React.createClass
 
   onStylesheetsChanged: (stylesheet) ->
     @refreshScrollbars() if @containsScrollbarSelector(stylesheet)
+    @sampleBackgroundColors()
     @remeasureCharacterWidthsIfVisibleAfterNextUpdate = true
     @requestUpdate() if @visible