Replying to child themes in builder

Hi John,
have a look at this post: http://ithemes.com/ithemes-and-child-themes-introduction/ You will see that it is very easy to create a child theme. There will be follow ups on it, detailing how to use child themes to modify the parent (Builder) to your likings.
You will need some CSS skills since all styling is done using stylesheets, and CSS code.
Hope this explains, if you have further inquiries, please don't hesitate to ask!Ronald

I am very curious about any limitations on child themes when using them in builder.  it seems like a fantastic way to get the look of other wordpress themes I like, but with the flexibility of builder in terms of layout.
however, I don't know what the requirements are for a child theme? can I use any other theme out there to borrow its design look?  how does wordpress know when to use the child theme vs the primary theme for design elements?
I am inches from choosing builder to be my platform, but just need to understand this more.  I am not very advanced in tems of wordpress theme customization in CSS, but am willing to learn.
thanks.