ds-site/ext_localconf.php

<?php

(function (string $extKey) {
    \TYPO3\CMS\Core\Utility\ArrayUtility::mergeRecursiveWithOverrule($GLOBALS['TYPO3_CONF_VARS'], [
        'BE' => [
            'stylesheets' => [
                $extKey => 'EXT:' . $extKey . '/Resources/Public/Backend/Css/',
            ],
        ],
        'RTE' => [
            'Presets' => [
                'default' => 'EXT:ds_site/Configuration/RTE/Default.yaml',
            ],
        ],
        'SYS' => [
            'fluid' => [
                'namespaces' => [
                    'f' => [
                        99 => 'DanielSiepmann\\DsSite\\ViewHelpers',
                    ],
                ],
            ],
        ],
    ]);
})('ds_site');