How to change font size?

How to change font size for my SMF forum theme? I see percentages which change only some parts of the text, can't find where to change post font, thread font etc...

Please just point me in the right direction, also what does xx%/yy% mean in regards to the font?

 /* SMFSimple.com Theme Style */

    body
     {
        background: url(../images/theme/bg.png) #f8f8f8;
   font: 78%/130% 'Oxygen',sans-serif;
   padding: 0px;
}

a:link, a:visited {color: #4D4D4D;}
a.new_win:link, a.new_win:visited {color: #4D4D4D;}

input, button, select, textarea
{
   font: 95%/115% 'Oxygen',sans-serif;
   color: #000;
   background: #fff;
   border: 1px solid #7f9db9;
}

textarea
{
   font: 100%/130% 'Oxygen',sans-serif;
}

.smalltext, tr.smalltext th
{
   font-size: 0.85em;
   font-family: 'Oxygen',sans-serif;
}
.middletext
{
   font-size: 0.9em;
   line-height: 1em;
   font-family: 'Oxygen',sans-serif;
}
.normaltext
{
   font-size: 1em;
   line-height: 1.2em;
}
.largetext
{
   font-size: 1.4em;
}

code.bbc_code
{
   display: block;
   font-family: 'Oxygen',sans-serif;
   font-size: x-small;
   background: #eef;
   border-top: 2px solid #999;
   border-bottom: 2px solid #999;
   line-height: 1.5em;
   padding: 3px 1em;
   overflow: auto;
   white-space: nowrap;
   /* Show a scrollbar after about 24 lines. */
   max-height: 24em;
}

.bbc_tt
{
   font-family: 'Oxygen',sans-serif;
}

@font-face {
  font-family: 'Oxygen';
  font-style: normal;
  font-weight: 400;
  src: local('Oxygen'), local('OpenSans'), url(../fonttheme.woff) format('woff');
}

#full_header 
{
   margin: 0 auto;   
   min-width: 764px;
   max-width: 2300px;      
}

#bar_social 
{
   background: rgba(255, 255, 255, 0.7);
   height: 50px;
   border: 1px solid #e8e8e8;
}

.ss_content_header
{
   clear: both; 
   overflow: hidden; 
   width: 100%;
   padding: 20px 0px;
   padding: 20px 0px;
}

.ss_logo 
{ 
   width: 48%; 
   float: left; 
   margin-top: 10px;
   padding-left: 20px;
}

.ss_user_area 
{ 
   width: 48%;
   float: left;
   margin-top: 15px;
}


.table_user 
{
   border-radius: 5px;
   background: rgba(00, 00, 00, 0.7);
   padding: 0px 10px;
   font-size: 12px;
   border: 1px solid #333;
   text-shadow: 0px 1px 1px #666;   
}

.user 
{
   color: #f4f4f4;
}
.user a 
{
   color: #F9BC06;
}

.ss_avatar_pad 
{
   padding-right: 15px;
}

This line:

font: 78%/130% 'Oxygen',sans-serif;

breaks down to:

• font-size: 78%;
• line-height: 130%;
• font-family: 'Oxygen', sans-serif;

If no font size is given explicitly, the browser's default value is used, which is actually 16px.