What is a theme?
In simple terms, a theme is the presentational layer. Regardless of the content management system (CMS), without a theme, all you have is content that looks very much like a Word document.
A theme generally consists of HTML markup, CSS, JavaScript, and media (images, video, and audio). It is this combination of technologies that allows the graphic designer to build something visually rich that can then be applied on top of the logic a web developer is building in Drupal. Sometimes, a web developer may be the person who implements the theme, but in most cases, you will hear the term themer or a frontend developer, which describes the person who fills that role.
However, before we dive right into creating a theme, it helps to have a better understanding of how to work with themes within the Appearance admin.