As the message at the top of that page says, the theming control panel is never themed. You can get to it directly by URL, e.g. http://localhost:8080/Plone/@@theming-controlpanel
. Then disable your theme on the 'Advanced settings' tab.
Alternatively you can disable Diazo for a page by appending diazo.off=1
in the query string, e.g. http://localhost:8080/Plone/front-page?diazo.off=1