Things will be lost with Core file changes which means physically changing the files of the theme itself, including the style.css. This is why it’s best to always use a child theme instead if one plans to make any core theme file changes, so that you can make theme updates when they come available. Updates only affects the parent theme but not the child theme.
However, anything you changed from the Theme Option settings (the theme customizer like colours), these won’t be affected.
Isn’t making websites fun! :cheer: