theme hook_suggestions not woking
I hope I understood your question correctly, that you want to apply theme on form submit.

theme_hook_suggestions is used when a template is being rendered. I think you don't need to save it on form submit. You just need to provide logic of selecting a template file in preprocess function of template.php file. Drupal looks for the suggested template file to apply to particular page/node etc

As a pseudo code, it would be

if(template_type is default)
   apply default type
else if(template_type is blank)
   apply blank type


For help, visit this link  https://www.drupal.org/node/1089656

Moreover, the example of preprocess function is as follows

function THEME_preprocess_page(&amp;$vars, $hook) {
  if (isset($vars['node']-&gt;type)) {
    // If the content type's machine name is "my_machine_name" the file
    // name will be "page--my-machine-name.tpl.php".
    $vars['theme_hook_suggestions'][] = 'page__' . $vars['node']-&gt;type;
  }
}