theme hook_suggestions not woking I have a content type called event_page. In event, I have a drop down for selecting templates. 

In the templates folder of my active theme, I have included my custom tpl.php files, such as node--event-page--default.tpl.php and node--event-page--blank.tpl.php.

I have used strreplace to remove 'node--event-page--' and '.tpl.php' to populate the types like bank and default from the files in the templates drop down. 

Now I am using hook suggestions to take these theme on form submit, but it seems to not working

if (isset($variables['node'])) {
  $node = $variables['node'];
  $field_template_list = $node-&gt;field_template_list-&gt;und[0]['value'];
  if ($node-&gt;type == 'event_page') {
    $variables['theme_hook_suggestions'][] = 'node__event_page__' . $field_template_list;
  }
}
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;
  }
}