Changes to non-default theme

So I have read that if you make changes to the default theme (twentyeleven) then you should make a child theme as when WP updates y9ou will lose changes. Does this also apply to non default themes? I am using the Big City theme. I have made changes to the stylesheet. Is it possible these changes could be lost? Can I just rename the theme instead of creating a child theme, and how do I do this?

A million thank yous!