Building a WordPress “child theme” is not all that difficult. WordPress has a very good write up on how (and why) one should create a child theme. But if all you are doing is adding or modifying the stylesheet, you may not need a child theme at all.
Check to see if the theme you are using includes the ability to add custom CSS. Something like this:
If it does, you can do perhaps all that you want without building a child theme. Child themes aren’t hard to build, but they do add overhead to your WordPress site’s load.
So much can be done with CSS alone, especially with a well made template that used CSS as it was intended to begin with. The whole idea of Cascading Style Sheets is to separate the presentation layer from the content and programming layers. For an idea of how much you can do with CSS, check out the CSS Zen Garden project where people are challenged to take a single HTML and CSS page and only modifying the CSS, make entirely different designs.
If you are a theme creator, please consider using HTML5 and CSS properly and adding into your theme the ability to add custom CSS. It will make your template used by a wider audience.