Hi,
Unfortunately it is not possible to add your own class to a link. Please feel free to raise a feature request for this on our feature request page: https://www.acymailing.com/feature-requests/
Instead of editing the html directly there is also a workaround on a per structure block basis. You can assign a block html id to your structure block and use that in your custom css to apply the styles you want.
So, if you assign "MyID" as html id you can add something like this to your custom css:
#MyID a {
color: #ffffff;
}