using child theme nothing changes

Oh sorry, just noticed that it looks like you have the same name here as in the parent?

Theme Name: NewStone

That has to be different – i.e. NewStone Child Theme

This child theme requires its parent theme, NewStone.” –> how come?

This message is normal.

Child stylesheet is working.
https://newsletter.mdsinsure.com/wp-content/themes/newstone-child/style.css

But there is too much content in it. The copy over of parent’s style is not necessary, the @import line already does that.