Are you submitting the page using Ajax? When are you trying to access the editor content in the server code? If you are submitting the page with custom client code, try calling the client API function saveContent() - e.g.:
var editor = $find("RadEditor_client_id");
If you are trying to access the content on the server in an early stage (e.g. OnInit event) it might still be unavailable because the LoadPostData method of the page has not been called yet.
The editor content area picks up the background color from its parent page. In your case the page background is gray, so you see the same in the editor. To fix this, you need to override the default behavior by using the editor's CssFiles collection (load alternate styles instead of the ones from the parent page) or override the page background by specifying an alternate content area CSS file (this will keep the rest of the styles in the content area intact). The second approach can be seen on our online demos - http://demos.telerik.com/aspnet-ajax/editor/examples/settingcontentareadefaults/defaultcs.aspx
Finally, to change the place where the editor looks for its dialog handler, you can try setting the DialogHandlerUrl property to "~/Telerik.Web.UI.DialogHandler.axd" and modifying your web.config registration to use the name Telerik.Web.UI.DialogHandler.axd
instead of Telerik.Web.UI.DialogHandler.aspx
(note the extensions - axd vs aspx). This is the last suggestion we give in the KB article about the error you saw - http://www.telerik.com/support/kb/aspnet-ajax/editor/error-web-config-registration-missing-the-telerik-dialogs-require-a-httphandler-registration-in-the-web-config-file.aspx
the Telerik team