Accordion CSS – Cant make it work Please help

I have purchased the accordion pro plugin, I want to reduce the size of the font to say 90%. I have tried all sorts of variations of CSS code and dont seem to be able to get it to make any difference.

Please can you tell me exactly what code I would need to type into the CSS box to reduce the font size. currently I have this in the box,

#accordions-599{}
#accordions-599 .accordions-head{}
#accordions-599 .accordion-content{font-size: 90% !important}

I know nothing about code! 😉

Please help