Hello,
Yes, if you edited original theme files. You need make all changes in custom.css file or Child theme to avoid this in future updates of the theme.
Here is a video tutorial how to create custom.css: https://www.youtube.com/watch?v=Qok2zRedRMY&feature=youtu.be.
Also read more about Child Theme:
https://codex.wordpress.org/Child_Themes
Regards,
Robert Hall.