By default, a Drupal site comes with Olivero.
To change your theme, you have a few options:
- Use Olivero as a base theme and make some modifications.
- Download a theme from Drupal.org.
- Create a custom theme from scratch.
- Use a front end framework.
- Choose a base theme.
The one you choose depends on your capability and needs. A more polished starting point may be faster and cheaper to implement, but limit your customizations. A custom theme could take longer than you think.
Bootstrap themes
Bootstrap is a mature front end framework that's great for content management systems.
There are several drupal themes to check out, here's one to start with: https://www.drupal.org/project/bootstrap_sass.
What does it take to make a custom theme?
A sense of style and some time. Really, you need to know about the content you need to display, and your technique for doing html/css templates. Drupal themes are made with Twig, which plugs variables from Drupal into your templates. If you have a good concept for a theme, and know how to write the code, you can make it happen.