Hello @kshoykhet888

Currently the theme doesn’t provide the option for changing the font style from Customizer.
But you can achieve this by adding custom CSS.
For that you need to go to Admin Panel / Appearance / Theme Options / Custom CSS.
In this section you will see a custom box to write custom CSS. In that box you need to copy and paste the following CSS.

h1, h1 a, h2, h2 a, h3, h3 a, h4, h4 a, h5, h5 a, h6, h6 a {
    font-family: 'Trebuchet '!important;
}
.main-navigation a {
    font-family: 'Trebuchet';
}
.site-title, .site-title a, .site-description, .site-description a {
    font-family: 'Trebuchet' !important;
}
body, button, input, select, textarea {
    font-family: Trebuchet !important;
}

Hope this will help to resolve your issue.
Let me know how it goes.

Best Regards!!