Yes, you will need to have ftp access to be able to upload new styles to your board. If you do not currently have ftp access, you will need to contact the board owner and ask if they are willing to grant you this access. If you are the board owner and do not have this, you are most likely on a free forum hosting service. If this is the case, you will need to contact your hosting company for information on how to customise your board.
Let us know if we can answer any other questions regarding this.