That seems involved. Unless I'm misunderstanding, you should be able to just go to Themes - [Your Theme] - global.css
body {
background: #fff;
color: #333;
text-align: center;
line-height: 1.4;
margin: 0;
font-family: Tahoma, Verdana, Arial, Sans-Serif; <-- Change this
font-size: 13px;
overflow-y: scroll;
}
Do CTRL+F to find other instances of 'font-family' in the global.css and make changes to them, too.
You can also use google-font, if you want.
Eg, google Font CSS would look similar to this:
font-family: 'Lavishly Yours', cursive;
font-family: 'My Soul', cursive;
font-family: 'Passions Conflict', cursive;
You will need to add the embed to the templates/[Your Theme]/Ungrouped Templates/headerinclide - for the example fonts above, they would look like this:
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
<link href="https://fonts.googleapis.com/css2?family=Lavishly+Yours&family=My+Soul&family=Passions+Conflict&display=swap" rel="stylesheet">
This calls forth the font if you don't use "default" style fonts like verdana or arial etc.
Google provides the CSS and <link> code once you've selected your chosen google font(s) as seen in the attached image.
ETA: this is the link to Google Font's Bengali selection:
Link