How to apply single theme to all webform in drupal 7
Easiest way would be using a module like Display Suite to change the page layout of your webforms on it's 'Manage Display' settings. Using a two column layout, you can arrange the content on the left side and webform on the right (webform being all fields you create on the webform part of the node).



If you don't want to use a module just for that, another option would be rewriting the webform node template yourself. For that, copy your theme's node.tpl.php and rename it as node--webform.tpl.php. Note that you would have to print out the correct variables on the markup yourself.