Piwik\AssetManager\UIAssetMerger\StylesheetUIAssetMerger::getCssPathsRewriter PHP Method

    private function getCssPathsRewriter($uiAsset)
    {
        $baseDirectory = dirname($uiAsset->getRelativeLocation());
        return function ($matches) use($baseDirectory) {
            $absolutePath = PIWIK_USER_PATH . "/{$baseDirectory}/" . $matches[2];
            // Allow to import extension less file
            if (strpos($matches[2], '.') === false) {
                $absolutePath .= '.less';
            }
            // Prevent from rewriting full path
            $absolutePath = realpath($absolutePath);
            if ($absolutePath) {
                $relativePath = $baseDirectory . "/" . $matches[2];
                $relativePath = str_replace('\\', '/', $relativePath);
                $publicPath = $matches[1] . $relativePath;
            } else {
                $publicPath = $matches[1] . $matches[2];
            }
            return $publicPath;
        };
    }