How do I change menu font?

I’d like to change the font of the menu to bold, same font and size, but bold. How/where do I do that in the css? Here’s an ectract of the stylesheet of the theme that I’m using. Thanks all!

/** Global **/

* { margin: 0; padding: 0; }
body { font-family: arial, verdana, georgia; font-size: 12px; background-color: #FFFFFF;  color: #c7c7c7; }
#outside_container { margin: 0 auto; background-color: #fff; border-top: 7px solid #FFFFFF; width: 1039px; }
#main_container { width: 959px; margin: 0 auto; }
h1,h2,h3,h4,h5,p { margin: 10px 0; }
h1 { font-size: 15px; }
a { color: #A1A1A1; text-decoration: none; }
a:hover { text-decoration:underline; }
.clear { clear: both; }
.left { float: left; }
.right { float: right; }
img { border: none; }

/** Header **/

#header { }
.header_top_text { padding: 5px 0; text-align: center; font-size: 20px; border-bottom: 1px dotted #ccc; color: #999; }
.header_text_cont { float: right; padding: 0 25px 0 15px; margin: 10px 0; border-left: 1px solid #ddd; width: 275px; }
.logo { float: left; margin-top: 15px; }

#menu_container { padding: 5px 0; border-bottom: 1px solid #FFF; }
#menu_container ul { list-style-type: none; }
#menu_container ul li { float: left; font-size: 12px; }
#menu_container ul li a { display: block; padding: 5px 15px; color: #999; text-decoration: none; }
#menu_container ul li a:hover { background-color: #333; color: #999; }

.current-menu-item a { background-color: #333; color: #999; }

.header_search { float: right; position: relative; }
.header_search input[type=text] { width: 284px; height: 28px; border: 1px solid #EEEEEE; padding-right: 30px; }
.search_icon { position: absolute; right: 10px; top: 10px; }

#slider_container { padding: 10px 0; border-bottom: 1px solid #E5E5E5; }
#single_container { padding: 20px 0; }

.home_blog_box { float: left; width: 306px; margin: 10px 20px 1px 0; }
.home_blog_box img { width: 306px; height: 215px; opacity:1.0; filter:alpha(opacity=100); }
.home_blog_box img:hover { opacity:0.8; filter:alpha(opacity=80); }
.home_blog_box_last { margin-right: 0; }
.home_blog_box h3 { font-size: 12px; font-weight: normal; color: #a4a4a5; }
.home_blog_box h3 a { color: #a4a4a5; text-decoration: none; }
.home_blog_box h3 a:hover { color: #333; }

.home_port_cont { border-top: 1px solid #E5E5E5; padding: 5px 0 25px; }
.home_port_box { float: left;  width: 306px; height: 387px; margin: 10px 20px 10px 0; }
.home_port_box img { width: 306px; height: 387px; opacity:1.0; filter:alpha(opacity=100); }
.home_port_box img:hover { opacity:0.8; filter:alpha(opacity=80); }
.home_port_box_last { margin-right: 0; }
.home_port_box h3 { font-size: 12px; font-weight: normal; color: #a4a4a5; }
.home_port_box h3 a { color: #a4a4a5; text-decoration: none; }
.home_port_box h3 a:hover { color: #333; }

#single_left { width: 628px; float: left;  }
#single_left p { color: #888888; }
.blog_box { padding-bottom: 10px; margin-bottom: 10px; border-bottom: 1px solid #E5E5E5; }
.blog_box p { color: #3E3E3E; }
.blog_box h3 { font-size: 15px; }
.blog_box h3 a { color: #c7c7c7; text-decoration: none; }
.blog_box h3 a:hover { color: #333; }
.blog_box img { width: 625px; height: 152px; }

.port_box { float: left; width: 306px; margin: 5px 20px 5px 0; }
.port_box_last { margin-right: 0; }
.port_box_divider { border-bottom: 1px solid #c7c7c7; margin: 5px 0 10px; }
.port_box img { width: 306px; height: 387px; }
.port_box h3 a { color: #c7c7c7; text-decoration: none; }
.port_box h3 a:hover { color: #333; }

.navigation { padding: 15px 0; }
.navigation a { color: #000000; text-decoration: none; }