Happy to assist as much as I can. So it sounds like you want to include 2 signup forms that go to the same list, correct?
If that's the case, you've got some options. It is possible to have multiple instances of the same signup form using the MailChimp widget. You'll either need to drag the plugin to a widget area in your site or use the built-in shortcode! Check out the different shortcodes, here:
Another alternative would be to use some combination of the MailChimp plugin and the embedded form:
How can I add my signup form to my website?: http://eepurl.com/gOGN
As far as sending one set of subscribers an E-book, depending on which form they use, I would use a MailChimp's "smart merge tags". We've got some pretty handy documentation on that, here:
How do conditional or smart merge tags work?: http://eepurl.com/hcV-/
Basically, in the Final Welcome email, you'd have something like:
*|IF:EBOOK|*Click here to download your Ebook!*|END:IF|*
Our support team might be able to explain that in a little more detail, so feel free to swing by our live chat or shoot us an email at help[at]mailchimp.com.
Now, as far as modifying the style.css file, that's a little more general. Essentially, you're just adding CSS to your stylesheet based on the classes and IDs that are set up for the plugin. My personal favorite way to do this is to use Chrome and right-click on the element I want to modify:
From there, click on "Inspect Element", which will bring up Chrome's built-in developer tools. This will let you make temporary changes to your site, on the fly and have Chrome render them for you.
You can then add whatever changes you've made in Chrome to your site permanently by adding that styling to your Style.css file in your WordPress setup. :)
Here's a general thread on this same topic:
Let us know if we can assist in any other way!