Where Everything Is
This part of the guide is to show you where the most common elements of the site are located. Remember, this site uses CodeIgniter Framework, so for well-documented additional support, please use their website and user guide: http://www.codeigniter.com/user_guide/
Styling & CSS
The site's styling is based off Bootstrap 3; Bootstrap 3 comes with lots of predefined CSS elements that you can use to create well presented websites with. You can view the Bootstrap documentation and examples here: http://getbootstrap.com/. If you wish to upgrade the Bootstrap version or make your own changes to it, you can find the files in the bootstrap/ directory.
The product also uses it's own custom styling to create the site layout. You can find the stylesheet file in styles folder.
Modify HTML / Frontend interface
The template files, also known as views, can be found in application/views/. These files contain the HTML of the site and modifying these files allows you to change the way the site presents data. These files also contain some PHP code so it's important to know what you're doing when editting these files.
The layout file can be found in application/views/layout/layout.php.
PHP Logic & Database Models
The controller files, which contain the site logic, can be found in application/controllers/ folder. These are PHP files that are used to control how the site works.
The model files can be found in application/models/ which contain the database logic.