From PHPBB用户手册

phpBB 3.0 is very customisable. Styling is one aspect of this customisability. Being able to manage the styles your board uses is important in keeping an interesting board. Your board's style may even reflect the purpose of your board. Styles allows you to manage all the styles available on your board.

phpBB 3.0 styles have three main parts: templates, themes, and imagesets.

Templates 

Templates are the HTML files responsible for the layout of the style.

Themes 

Themes are a combination of colour schemes and images that define the basic look of your forum.

Imagesets 

Imagesets are groups of images that are used throughout your board. Imagesets are comprised of all of the non-style specific images used by your board.