Tara Thompson wrote:
sorry, i can not located it. i did use firebug but i can not see the direct path
My bad 
You should create a file called rt_metropolis-custom.css and place it in your ../templates/rt_metropolis/css folder.
Then add the code there.
Some resource information for you.
We recommend that you create custom CSS files and simply override the original CSS for any content elements you desire.
Please see these links to better understand how to create your own custom CSS file for our newer LESS templates:
LESS CSS
Your first CSS-Less custom file or CSS custom file
Here's a tip for you ->
Developing & Debugging LESS and CSS