Different stylesheet used

Dear developer,

I found the main page for forum and the subforum is using a different stylesheet and I don’t know how to deal with the issue. I would like to apply the stylesheet of subforum on the main forum page as well. What should I do?

Main forum: https://www.howtostudycantonese.com/forum/

Sub forum: https://www.howtostudycantonese.com/forums/forum/cantonese-writing/

Thank you for reading and your help!