There are no 'sample' themes, there is only the 'Basic' theme. The marketplace
has several themes which you can use, including a few from Sitefinity.
The basic theme (installed by default) includes general styling on all
widgets and components. The Sitefinity free samples from the marketplace are not complete. (The university template for example has no styling for forms/ecommerce).
My guess is they aren't included because they are showcases/starting points but in no way supported as the basic theme is.
Not sure what you mean by the API isn't included in the installer, all the logic is there and you can code away regularly or fluent. Documentation and samples are included however in the SDK.
The SDK can be downloaded by everyone who has an account, since you're able to post on the forum you have an account so you should be able to click this link
and download the latest SDK (v4.3.1885 November 10 2011). As far as I'm aware, even trial users can download the SDK.
Since you've created your own theme, the basic theme no longer applies. So all styling to any widgets is lost, including the ecommerce widgets.
My suggestion is play around with master pages and widgets first before creating your own theme, that way you get a feel for the power, flexibility and design flair built into the system and then port it all over to your own css...
You mention that you've initiated a 30-day trial of the 'Standard Edition', the standard edition doesn't include ecommerce in it's license, its a paid add-on. Not sure what the limitations are with regards to a 30-day trial, but if it's indeed ecommerce you'd like to test out consider downloading a professional license.