If you want to change the css only for the homepage, you should use .home class before your css selectors.
I.e.:
- Code: Select all
.home h1 { .. }
for all h1 headers on the frontpage.
Regarding the child themes, of course you can use it, but themes like John uses our GavernWP framework and there will be small difference, please notice that in the John (parent) theme style.css file there's no css rules at all (all css are placed in the John/css directory), so if you want to change css files or add your own css file, you should copy these files into John-Child/css directory (only the files which you want to change, and a the bottom you can add your own css rules).
Generally in you Child Theme all css files are loaded from the parent but if you have file with the same name and in the same directory i.e. John-Child/css/override.css, this file from the child directory will be loaded.