Shopify has a bunch of Form Theme Tags to creates an HTML <form> element with all the necessary attributes (action, id, etc.) and <input> to submit the form successfully. Handy. One undocumented feature is the ability to add a custom class to the form.

Not that this is really needed (since you can just wrap the form tag in a parent element) but should be a purist just add the class as a parameter:

{% form 'activate_customer_password', class:'foo' %}

Now that's cool and all but the real fun comes when you realise your can do something like this:

{% form 'contact', class:'foo', action:'more-foo', data-type:'super-foo' %}