If you are going to customise your Divi theme, then it is best practice to do it using a child theme so that when you update the original Divi theme, any changes you make are not overwritten by the updated master theme. You can then add and edit any of the files into the child theme and know that when the next release of Divi comes out you can update without reverting back to a standard Divi theme.
If you want to customise you Div theme, then the best way is to create a child theme, and it is very easy if you have FTP access. Just follow these steps.
1) Create a new theme folder (say divi-child) which sits in your wp-content>themes folder in your directory.
2) Add a file called style.css to the divi-child folder
3) Add the following code in the style.css file;
Theme Name: Divi child theme
/* YOUR CSS EDITS GO BELOW THIS LINE AND AND CSS RULES YOU MAKE HERE WILL OVERIDE THE MASTER DIVI THEME CSS RULES */
4) Upload the files using FTP – There are several ways to upload files to your server. I use coffeecup HTML editor, but you can also use the file manager in your hosting cPanel, Notepad++, Filezilla amongst others. You can also just zip up the files and upload and activate directly in your WP dashboard (appearance>themes>add new), just like you uploaded the Divi theme.
5) Activate the theme in your WP control panel.
And that’s it, your theme is now running on the child theme and pulling in the info from the main Divi theme. Just add your custom CSS into the style.css file in your child theme and it will update on the site. Next time there is an update to the Divi theme you can update the master Divi theme and leave your child theme, and any customisation you did in it, unaffected.
As you can see there is also a footer.php file that I added so that I could edit the footer of my theme to remove the links to Elegant Themes. You can copy and paste any file from the main Divi theme into your child theme (get the hierarchy right) and edit as needed.
There is an excellent post about this on ET @ http://www.elegantthemes.com/blog/resources/wordpress-child-theme-tutorial