Changing Font Size and Type

Hello there.
I want to modify my wordpress post font size.

body { background: #0f0608 url('images/bg.jpg') no-repeat top center; text-align: center; line-height: 21px; font-family: Arial, Verdana, sans-serif; font-size: 15px; color: #000000; }
to
body { background: #0f0608 url('images/bg.jpg') no-repeat top center; text-align: center; line-height: 21px; font-family: Arial, Verdana, sans-serif; font-size: 15px; color: #000000; }

When I do that in style.css the font size is changed but the problem is that the font is bigger everywhere my blog, I only want to change it on post. (where the article is written)
I am using Glow WordPress Theme.