How do you style an individual webform?
If you just need to apply CSS styling, then if you view the source of the webforms you will see they have container divs with classes that contain an ID number. This ID will allow you to directly target a specific webform.

If you mean you want to add your own divs etc. and template different webforms, then the best way to do this is by creating a specific template file for it.

In your theme folder you can add a template file such as:
webform-form-[nid].tpl.php you can get the ID by viewing the source of your form, it displays in one of the forms parent divs.

As your base you can use the default template from the webforms module. Goto (or other location where you have installed the webforms module):

/sites/all/modules/webform/templates/webform-form.tpl.php


and make a copy of this.

I have themed webforms that appear in blocks in the following way too:

block--webform--client-block-112.tpl.php