ranahusam Hi. This may not be accurate, but lets try it since we cannot check Your site. This setting is supposed to be configured in template settings, since it's overwritting most of the css classes, but try add this to css:
body .item-page {
font-family: 'Font-name', sans-serif !important;
}
Still would advice to change it in template's configuration, but if for some reason You can, than this should work.