The default font for the portal 

An admin can set up a default font for the agent portal from Admin > Account > Helpdesk Settings. This is the font that your agents will use while composing messages/replies to your customers. We have included more font family and font size options. You will also be able to see a preview of how the font would look like. 

Agent’s font preferences

If you want your agents to change the font format, then you can enable the ‘Font preference for agents’ option under Admin>hepdesk. 

The agents can choose to change the font family or font size from the reply editor under the ticket details page while replying to the tickets. 

When the agents click on the icon 'A', they will be able to view and choose the formatting options as shown below :

Rich text formatting in Freshdesk Support Desk mobile app:

When typing a response, agents can tap the ‘A’ text icon in the message field to open the formatting toolbar. They can choose to select an option from the toolbar, preview what the message exactly looks like before sending it.

With this, you can bold your text, italicize it, link it to a webpage, add lists, and so on.