Hello Ruud,
There is no need for additional CSS styling in order to use the wp-dark theme. Please make sure that you have upgraded to the latest CSS files in your project, as the wp-dark theme was recently released and you might not have the correct CSS rules. If the problem still persists, please extract a small runnable sample that we can look at and advise you further.
Regards,
Kiril Nikolov
Telerik
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