Hi Bram,
Thank you for your interest in Syncfusion products.
Yes, we can apply the custome style for the TOC levels. Please refere the below documentation link to create Custom style for TOC levels.
http://www2.syncfusion.com/ug_63/docio/TableOfContents.htmlPlease try using this and let me know if you face any issues.
Best Regards,
Bhuvana