Hi @incsol
With the terms, the HTML can be added into the payment terms box on the invoice. You can set a default version by going to Account Settings >> Invoice Customisation which will then set it by default (but you can still edit it from invoice to invoice).
We support a number of different HTML tags - a full list can be found here: Rich Content Support
You can add HTML using these boxes if you wish and if you add IDs or classes, you can then customise this through the CSS customiser.