Update and childtheme

I made my new site with the catch kathmandu free theme. I made some changes by using the css.box. I am verry happy with the results. I read in the forum that changes will disappeare when not using the childtheme I'm using direct admin. Is it necessary to update the theme? And if Yes do in need to make à childtheme? When in need to make an childtheme where do i find information to make one. My site www.ergotherapieveltman.nl