If you are working on a website for commercial use, it's probable that you will have designs or at least design guidelines to follow. It is possible to create your own custom themes for Drupal to achieve the exact look and feel you need.
Creating custom themes is beyond the scope of this book, but before we conclude this topic, it's worth introducing a couple of other concepts important to Drupal theming when you take the next step.
Drupal themes are designed to be extendible so that you can build a theme of your own that only changes some small aspects of a parent theme. It's not best practice to edit the content of any part of Drupal core, but you may, for example, want to modify the CSS of the Bartik theme for your own purposes.
In this scenario, you would make a subtheme of the Bartik theme and then edit the CSS in your version of the theme. As mentioned earlier, some of the other themes provided in Drupal core such as Classy is not designed to be...