Add Customized Files “/Includes” Folder to WordPress Child Theme

I recently updated my SystemSpecialist.NET Blog theme and of course lost some modifications I made to the theme, because they where overwritten. Since I had my notes about the modifications I didn’t worry too much, but also wanted to fix this for the future. I’m not a WordPress hero, so I went Googling for some answers. The answer was simple. I had to create a WordPress Child Theme (http://codex.wordpress.org/Child_Themes). The concept is simple because every file found in the Child Theme overrules the file in the Parent Theme. But for some reason I wrestled with the /includes/ folder. I have a customized function in one of the theme’s php files, ...