Customizing look

There are several items I’d like to modify but haven’t a clue where I might find them in the blue.css.
– change size, color & placement of the social icons in the header
– add phone number in the header area
– reduce the width of the header (currently takes up more than 1/2 the width of the page, this is where the social icons & navigation tabs are also located)
– change background color of footer area

I understand I need to create a child theme to place any changes into.

Appreciate help