XF 1.4 Importing a new style


Active member
I'd like to import a new style and being a complete novice, I'm stuck with what to select here:
Please could you explain what Child of Style means and whether to select a parent style. And when would you select Overwrite style?
Can I go back to the default Xenforo style if I don't like the import? If so, how?
Thanks for your help.