Look in includes/templates/your_template_name/css/stylesheet.css, near the top where it says...
Code:
body {
margin: 0;
font-family: verdana, arial, helvetica, sans-serif;
font-size: 80%;
color: #000000;
background-color: #e5edf5;
}
Change the font-family defiinition to whatever will work for your site.
Of course this doesn't ensure that site visitors will have any of those fonts installed in their computers, but if they're going to a site they expect to be in Hindi, then they probably will have one of those fonts.
I just did a Google search on "Hindi fonts for the web" and got a number of apparently useful hits.
Rob