In the classic Theme (Themes/classic) you will find a sample file. It contains a few comments, and a basic outline for that theme. The elements in it all have particular meanings and uses, but most of them are just for display purposes. The name is possibly the most important thing in this file. With it you can specify the name used for your Theme (fancy that!) throughout SMF, at least by default.

The author and website are there just to clarify things, and are not actually used when displaying the name of the theme. It is still recommended that you specify them, such that people can more easily tell which Theme is which.

Both the layers and templates options are very important; but in most cases, they can be left at what you see in the example file. The layers option defines the default set of layers to display on every page, comma delimited. For example, to show the 'site' layer and then the 'main' layer, you would put 'site,main' under that option. The templates option specifies which templates should be loaded on every page view, such as 'index' or 'index,Important,Admin'. However, that is fairly advanced. In most cases, 'main' and 'index' are fine.

The based-on option is currently reserved for later use, but you can put 'default' there to specify that your Theme expects the images_url and theme_url to be set to those of the default theme. In later versions of SMF, this feature may give you the option to load missing templates from a Theme other than default, which is currently always the case.