There are 3 elements to a theme (but only the first two are required): page templates, styles and scripts.
If you copied an existing theme, templates will be found in templates folder. They are html documents with additional benchpress templating syntax and they define the bare layout and contents of the page. You can find some basic information on templates here and there is also benchpress documentation that will probably be useful.
Styles are just less css files (located in less folder usually) and define the look of the page (and can also define layout). You can learn more about less here or just by googling some tutorials