Hi There,
This is not our phpBB style issue. It’s all about your language pack issue. phpBB not found any stylesheet in your nl folder.
However, you could fix it to follow the steps below:
1. Open overall_header.html
2. Comment this code <link href="{T_STYLESHEET_LANG_LINK}" rel="stylesheet">
3. Save the change
4. Clear your phpBB cache and check the change
Best Regards,
ThemeLooks Support Team