Custom theme lacking backend widgets page

I’m working on a site for a local company who lost their (far more qualified) developer before they were able to launch. They are using a custom designed theme which doesn’t include a widgets page on the backend. I am comfortable adding widgetized areas into the theme itself but I don’t know how to include the page where one can actually manipulate placement of the widgets.

Is there a relatively simple way for me to fix this? Or am I SOL?