Hi Sonam
You can do this in a number of ways.
If you are using Marketo templates, you should be able to define this either section-by-section or on the entire template. When you create a new email from an existing template, click "edit code", and change the font references to be Din Pro throughout the HTML and CSS (do a quick find, that will make it easier!) Once you are happy with it, you can save that email and re-use it ongoing. I think theoretically you could also create the font as a "global variable" within a template, which would allow users to change this on an email-by-email basis. But be careful, if they get it wrong, it will break the whole email.
If you are creating templates outside of Marketo and copying and pasting in the code (as we do) then you will define the fonts in the HTML/CSS of the file you create.
Overall, be careful with web fonts - not all email clients support them. You will need a suitable fallback. Litmus has a great guide on this: https://www.litmus.com/blog/the-ultimate-guide-to-web-fonts/.
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