I would like to create a trigger so that when a new site is created, a login design is automatically set for the site. any help?
There isnt a way right now as far as I can see. Adding triggers like this is not very difficult - you could contact Squiz to ask for a quote
The other way is to modify the system login design (which is used as the default).
You can do this with a trigger if you always want to set the same login design:
Event: Asset Created
Condition: Asset is of Type Site
Action: Set Design / Login Design / Select your Login Design
Works like a charm. 