I’m doing the same thing for my themes. I’m currently using twenty fourteen as a base for my child theme. What I do, I create 2 folders, namely css and functions. Say for instance I putting custom code in the footer, I will create a footerstyle.css in my css folder and a footer-functions.php in my functions folder, and register/enque these in my main functions.php template. All styling and coding goes into those templates that is specific to my footer. I also make a note in my footer-functions.php where exatly this code must go and how it should be used in my footer.php template. I do this for widgets functions, header functions, general functions etc. My theme has got about 10 of these functions template. It is also easy if I need to change my code for something specific, for instance, if I need to change something in my footer I know I need to go to my footer specific function template. All other theme related styling and funtions goes into the default style.css and funtions.php. So after all said, when twenty fifteen is released, I copy my functions.php, functions folder and css folder to my new child theme for twenty fifteen, delete all the twenty fourteen specific code from my functions.php template and do all the minor tweaking and I’m good to go. Hope this helps
