Update admin_themes.php

A simple explode doesn't allow for an option value to contain an equal sign. With this change, that cares about equal signs a little more, theme options can contain HTML. Consider the following theme:

class Theme_Xyz extends Theme
{
  ...

  public function __construct()
  {
      $files = glob(FCPATH.SHARED_ADDONPATH.'themes/xyz/img/option-images/*');
      $image_options = array('=None');
      
      foreach($files as $file) : 
         $clean_path = str_replace(FCPATH, '/', $file);
         $image_options[] = $clean_path.'=<img src="'.$clean_path.'" style="height:100px;">';
      endforeach;
      
      $this->options = array(
         ...,
         'image' => array(
            'title' => 'Image:',
            'description' => '',
            'type' => 'radio',
            'default' => '',
            'options' => implode('|', $image_options),
            'is_required' => false,
         ),
         ...,
      );
  }
  ...
}