How to override an existing theme without subtheming it?

Question

Does anyone know how to overload/override an existing theme with one's own .css and .tpl.php file, without subtheming it?

Answer 1

You can call for your css

<?php
function mytheme_preprocess_html(&$variables) {
  drupal_add_css('http://fonts.googleapis.com/css?family=News+Cycle', array('type' => 'external'));
}
?>

and for your tpl file

<?php
/**
 * Implements hook_theme_registry_alter().
 */
function my_module_theme_registry_alter(&$theme_registry) {
    // Defined path to the current module.
    $module_path = drupal_get_path('module', 'my_module');
    // Find all .tpl.php files in this module's folder recursively.
    $template_file_objects = drupal_find_theme_templates($theme_registry, '.tpl.php', $module_path);
    // Iterate through all found template file objects.
    foreach ($template_file_objects as $key => $template_file_object) {
        // If the template has not already been overridden by a theme.
        if (!isset($theme_registry[$key]['theme path']) || !preg_match('#/themes/#', $theme_registry[$key]['theme path'])) {
            // Alter the theme path and template elements.
            $theme_registry[$key]['theme path'] = $module_path;
            $theme_registry[$key] = array_merge($theme_registry[$key], $template_file_object);
            $theme_registry[$key]['type'] = 'module';
        }
    }
}
?>

Answer 2

It is possible, by simply editing the .info file in the folder of that theme, and edit/overwrite the relevant tpl.php files in the theme-folder, however I wouldn't recommend overwriting a core/existing theme. When the Drupal Core or the theme is updated, you lose your modifications, because your files are being overwritten by updating.

The other way is implementing hooks like user2754599 said.