Font problems in Sakura

Hi, I am using the Sakura template and I waned to change the fonts. I have tried changing them in the CSS but when I preview my post, some of it is in the new font and some of it is in a different font entirely. Can you tell me why this is and what I can do about it? I have heard about ‘closing’ the font tag. What does this mean and how do I do it? Thank you.