TYPO3 Core

Since the new web component based NewContentElementWizard custom included CSS via TBE_STYLES is not included anymore.
This leads to broken preview icons, if a custom IconProvider was used. E.g. Mask uses FontAwesome Icons, but the CSS is not included from
$GLOBALS['TBE_STYLES']['skins']['fontawesome_provider']['stylesheetDirectories']['css'] = 'EXT:fontawesome_provider/Resources/Public/Css/';

The backend.css file seems to be hard-coded:

Build/Sources/TypeScript/backend/new-content-element-wizard.ts:343

    // Load shared css file
    const link = document.createElement('link');
    link.setAttribute('rel', 'stylesheet');
    link.setAttribute('href', TYPO3.settings.cssUrls.backend);
    this.shadowRoot.appendChild(link);

typo3/sysext/core/Classes/Page/PageRenderer.php:2244

    protected function addGlobalCSSUrlsToInlineSettings()
    {
        $this->inlineSettings['cssUrls'] = [
            'backend' => $this->getStreamlinedFileName('EXT:backend/Resources/Public/Css/backend.css'),
        ];
    }