Themes reside in own folders, where the foldername equals the theme name. Such folder content constist mainly of template files (*.tpl) and style sheets (*.css). Do not be too impressed about the plenty of, if you see it the first time. Let have a look to:
The themes folder has subdirecories, like default, like nonparail etc. The content of such subdirectory consist of tpl and css files, they together form a theme. While template files are resposible for the xhtml content feeded by the phpWebSite modules, css files have the mission to style. To develop an own theme, best first copy (duplicate) the default theme (folder) to an own (in this example just happened with the theme name own). The reason is quit simple: to exercise layout changes, keep the default theme intact anyway.
1.1.2.Tools
As mentioned, tools are required to have a transparent exercise field. There may be a lot, but I only suggest the use of the FireFox (FF) WebBrowser. FF offers the abilities to add plugins and the two prefered are FireBug and DomInspector. Most people know, Firefox and the mentioned plugins are available for free. One remark, I prefer the Opera Webbrowser to visually verify layouts because he seems to be the strictest one in true regarding offical standards. But FF has autstanding features for tracing, so - best - use both. And also, what was the name of ... IE?
1.1.3.Using Firefox
Everybody just using Firefox need the plugins FireBug and DomInspector, all others may download and setup that web browser before.
At FF menu -> Extras -> AddOns the both plugins may be received.
1.1.4.Own theme preparation
Assuming the default theme is just copied to something like own, the theme has to be allocated to your site within phpWebSite -> ControlPanel -> Administration -> Layout:
The theme change has to be confirmed within a time limit, else the previous theme assignments remains in effect:
In the most right corner quit down the FF window, so see a gray symbol like for FireBug.