Thanks for contacting us.
Based on your input, you'd want to use the Sitefinity template manager to reuse navigation, footers and other controls. The template editor is a lot like a page, so you select a master page, a theme and then drag controls onto the page. When you save the template, you then make a page and select that template and all the controls pop up. For more info on our template editor, please watch this video
. More info on the template manager is also found under Working with Page Templates in our User Manual
As for your theme issue, we'd be happy to take a look and see why overlay mode is not working. Can you include your master page and theme in your response? Likely some of your CSS classes are not the same as ours, which could cause this error.
I hope that I have clearly answered your question. Feel free to contact us if you need more help.
the Telerik team