Where can i find the stylesheet?

Where do i find the stylesheet so i can customise the front end look. were trying to decide between 2 different types of booking systems and i want to see how easy it is to change the style of your appointments plugin. we have it installed on this page: http://www.allaboutahealthyou.com.au/contact-us/
and we want it to have nicer buttons, fonts and a better looking calendar. Is there a way via wordpress i can do this or do i have to do it in dreamweaver or similar?