Ms Bookings Form

Hi everyone

I'm having a little problem with my ms bookings form. It makes use of a media query (max-width: 1020 px) where it sets a max-width of 420px to the 'mainContent'. Is there any way to overwrite this media query? I've tried adding my own css, but it doesn't seem to work. How can I fix this issue? 

Thanks in advance,

Gentian