Editing the files directly is only required if you would like to make very deep customizations such as to the structure or editing the colors
Here is a breakdown of the theme structure:
CSS files - (found in theme_directory/css/) - The CSS files are used to style the theme, this includes colors, fonts, spacing & design.
Image files - (found in theme_directory/images/) - Here you will find the images of the default theme. Such as images used for backgrounds, the default logo, partner images etc.
header.php - This is the file used at top of the theme, where we can find the code for Menus both Top Right Menu and Main Menu. It contains all of the structure for the header of the theme.
footer.php - This is the file used at bottom of the theme, where we can find the code for footer menus and copyright. It contains all of the structure for the footer of the theme.
sideabar.php - This file is use to hold all the widget at the rightside of the theme. But this area is hidden if the page is set as one column layout.
page.php - This file controls the layout of the sub-pages of the theme.
index.php - This file controls the layout of the homepage of the theme.
one-column.php - This file controls the layout of the page when one-column layout is selected on the page creation wizard. Specifically this is the same as page.php but without the sidebar.
functions.php - This file loads all of the functions of the theme (menus, widgets, shortcodes). It also allows you to create and add your own custom functions.