change font to san serif font?

How can I change the font of all text to a san serif modern font like the one below?

 

Hello,
Paste the following code in your Custom CSS
(Appearance -> Theme Options -> Styling Options -> Custom CSS

Code:

body,p,a,h1,h2,h3,h4,h5,h6,span,.infotag,.textwidget,li,ul,label{
font-family:sans-serif ! important;
}

Put your font-family inplace of "sans-serif".
This will solve your issue.