Hi,
the plugin does not use the jdownloads_custom.css file so that its code has to be duplicated either into a new own file only for the plugin or integrated into joomla template user css. Duplication could be avoided if you integrate the custom.css file into the plugin itself.
At line 139 of plugins/content/jdownloads/jdownloads.php should be added
$custom_css_path = JPATH_ROOT.'/components/com_jdownloads/assets/css/jdownloads_custom.css';
if (JFile::exists('$custom_css_path')) {
$document->addStyleSheet(JURI::base().'components/com_jdownloads/assets/css/jdownloads_custom.css', "text/css", null, array());
}
I did it and it worked – but it might be lost by a future update of jD component. So I copied that file into a backup file to be able to reintegrate my changes into a new version of that file.