If you look at Planetfall_customizing, you can see basically how the page is put together, at least as far as the big items are concerned. Template Overview describes how Tiki pages are assembled with template files. To modify the Planetfall theme, you may need to edit the templates/styles/planetfall templates. As the template overview says, tiki.tpl basically holds everything; other template files are included in this template, depending on the content to be displayed, etc.
The template files use Smarty syntax, which is simpler than normal PHP.
What to modify and how depends on what modifications you want to make, so if you describe your plans more specifically, maybe we can help here.