framework/base/Theme.php
public function applyTo($path)
    {
        $pathMap = $this->pathMap;
        if (empty($pathMap)) {
            if (($basePath = $this->getBasePath()) === null) {
                throw new InvalidConfigException('The "basePath" property must be set.');
            }
            $pathMap = [Yii::$app->getBasePath() => [$basePath]];
        }
        $path = FileHelper::normalizePath($path);
        foreach ($pathMap as $from => $tos) {
            $from = FileHelper::normalizePath(Yii::getAlias($from)) . DIRECTORY_SEPARATOR;
            if (strpos($path, $from) === 0) {
                $n = strlen($from);
                foreach ((array) $tos as $to) {
                    $to = FileHelper::normalizePath(Yii::getAlias($to)) . DIRECTORY_SEPARATOR;
                    $file = $to . substr($path, $n);
                    if (is_file($file)) {
                        return $file;
                    }
                }
            }
        }

        return $path;
    }
public function applyTo($path)
    {
        $pathMap = $this->pathMap;
        if (empty($pathMap)) {
            if (($basePath = $this->getBasePath()) === null) {
                throw new InvalidConfigException('The "basePath" property must be set.');
            }
            $pathMap = [Yii::$app->getBasePath() => [$basePath]];
        }
        $path = FileHelper::normalizePath($path);
        foreach ($pathMap as $from => $tos) {
            $from = FileHelper::normalizePath(Yii::getAlias($from)) . DIRECTORY_SEPARATOR;
            if (strpos($path, $from) === 0) {
                $n = strlen($from);
                foreach ((array) $tos as $to) {
                    $to = FileHelper::normalizePath(Yii::getAlias($to)) . DIRECTORY_SEPARATOR;
                    $file = $to . substr($path, $n);
                    if (is_file($file)) {
                        return $file;
                    }
                }
            }
        }

        return $path;
    }
Converts a file to a themed file if possible.
If there is no corresponding themed file, the original file will be returned.
@param string $path the file to be themed
@return string the themed file, or the original file if the themed version is not available.
@throws InvalidConfigException if [[basePath]] is not set