Example rewrite to disable Magento from falling back to base/default

BaseBlocker.php

<?php
class Vendor_BaseBlocker_Rewrite_Mage_Core_Model_Design_Package extends Mage_Core_Model_Design_Package
{
    /**
     * Magento's function with additional condition to around falling back to base/default
     *
     * @author Wenjiang Xu <wenjiang.xu@ampersandcommerce.com>
     */
    protected function _fallback($file, array &$params, array $fallbackScheme = array(array()))
    {
        if ($this->_shouldFallback) {
            foreach ($fallbackScheme as $try) {
                $params = array_merge($params, $try);
                $filename = $this->validateFile($file, $params);
                if ($filename) {
                    return $filename;
                }
            }
            // This condition is added to allow certain files to fallback to base/default
            if ($this->shouldFallbackToBase($file)) {
                $params['_package'] = self::BASE_PACKAGE;
                $params['_theme'] = self::DEFAULT_THEME;
            }
        }
        return $this->_renderFilename($file, $params);
    }

    /**
     * This function is a white list, return true to allow file to fallback to base/default
     *
     * @param $file
     * @return bool
     * @author Wenjiang Xu <wenjiang.xu@ampersandcommerce.com>
     * @author Yousef Cisco <yc@amp.co>
     */
    protected function shouldFallbackToBase($file)
    {
        // Allow fallback in adminhtml
        if ($this->getArea() != self::DEFAULT_AREA) {
            return true;
        }

        $exceptions = array(
            'vendor',
            'paypal',
            'sagepay',
            'payment',
        );

        // Allow exceptions
        foreach ($exceptions as $exception) {
            if (stripos($file, $exception) !== false) {
                return true;
            }
        }

        return false;
    }
}