Email templates, at this point, are a bit of a mess. Most of them rely heavily on inline styling (i.e. <a href="..." style="color: #123456" .../>). For where this is the case you can't do much more than editing each template individually and change its style (i.e. by removing the color so it behaves more 'naturally') via Settings -> Technical --> Email -> Email Templates (requires you to enable the Developer mode first: https://www.odoo.com/documentation/18.0/applications/general/developer_mode.html). All in all I would assume you will be facing a hard time when trying to modify everything to look perfect with an Odoo Online plan due to the fact that many mails (or there contents) are also generated dynamically, which would have you edit the source to address those. Also with a custom plan, this is not a trivial task (not too difficult, but certainly a lot of work).