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From the title I just wanna know the right way to change the default font 

In global.css mostly. There are multiple definitions that you need to change depending.

body {
background: #fff;
color: #333;
text-align: center;
line-height: 1.4;
margin: 0;
font-family: Tahoma, Verdana, Arial, Sans-Serif;
font-size: 13px;
overflow-y: scroll;
}

#container {
color: #333;
text-align: left;
line-height: 1.4;
margin: 0;
font-family: Tahoma, Verdana, Arial, Sans-Serif;
font-size: 13px;
min-width: 990px;
}

etc.

the question is I want add a custom font where I put it ?

Please take a look at the replies here.