I want to change the sidebar style like yours.
where can I change the style?
should I make a child theme and customize it?
There is no option or setting to achieve such a display.
But you do not have to use the child theme because it could be accomplished with a custom CSS code.
In that case, you should contact your web developer and place the code he/she prepares in Betheme -> Theme options -> Custom CSS & JS -> CSS.