I'm doing a site for someone and I've been learning so much about WP over the past week from nothing, it's great.
I was told I need to create a child theme before making changes to my twenty ten theme. However I only found that out recently, after making quite a few changes to the theme, but only using the style.css page which i've added or changed rules to - I am avoiding touching the php files : So I've changed font and background colour/size, hidden meta tags, added an item to the navigation, and all pages have relevant text / images in them.
My question is, if the client decided to hit 'update' one day, what exactly would happen ie which data would be lost? Does it mean all content in pages menu items etc will revert to the original, or would they only lose all style changes ?