Here you can select a theme. By selecting a theme, the theme details are stored in the session and applied to the template when rendering the resulting page.
Basically, the theme you select will do the following:
config/theme.php.<html> element, if defined.You edit the settings for each theme in the configuration file config/theme-selecotr.php.
The view for the theme selector is in view/theme-selector/index.tpl.php. There is no need to edit this file.
The code that applies the details from session, to the template during rendering, is config/routes/custom.php. There is no need to change that file.