Hi Olly sorry if this is a completely stupid/simple question!
I have added filters into the functions.php to customize my html emails to look like this:

I would like to add text to the top and bottom of the box. I have tried to just edit the template and put it into my theme file but some email clients strip these details and some don’t (so if I go directly onto apple icloud website the text is missing but if I use my iphone to access the email I can see the text).
I also don’t want to just edit the template so I don’t miss out on future updates etc.
Are there any filters available to add text to the top and bottom of the email? (I’m sure this is very simple but I don’t know how!)