When you make changes for the my_changes addon, don’t forget to install your hooks into the var/themes_repository structure so that when you load a new theme (or create one) that your changes will be applied into the new theme. Your hooks/templates should be installed in both the ‘basic’ and ‘responsive’ themes with any related css done appropriately for each.
Otherwise, if you do load a new “theme”, it will not have your changes in it.
Unfortunately, the term “theme” is overloaded in cs-cart. It is used to mean both a whole layout/presentation and simply style changes. So be careful when applying changes to “theme”…