Add highlight to CSS

Hello,

I know this is a seriously rookie question, but I’m not quite sure what to search for.

All I want to do is make it so our website’s text has highlight around it (white text over black highlight). Since manually doing it is absurd, I imagine I’d want to change the p class in style.css. That and Optimizer doesn’t offer this as an option.

Is there a way to add
background-color:#000000; ?

And if so, where? I thought placing it in here:

body{ background:#ffffff; margin:0; padding:0; font-family:Arial, Helvetica, sans-serif; font-size:16px; color:#999;}
.center{ width:85%; margin:0 auto;}
img{max-width:100%}
h1, h2, h3, h4, h5, h6 { line-height: 1.7em;}

/*Full width pages*/
body .no_sidebar{ width:100%;}

but no dice, am I missing something? Has someone already asked this question? is it glaringly obvious?

again, sorry to ask such an inane question, but I’m not quite confident with html/css to completely understand how optimizer works under the hood.

regards,
D