HEADER – change size of the title of site (andlogo) the menu

Hello
I would need to get the title of the site + a logo in the header banner; but I do not fond the file concerning the size (in the style .css there is nothing it is totally empty and in the header.php nothing also)
and how to change something or add somethng without deleting what I dd not found ?)
I would need to get the title + logo and after the menu at the right place.
I created a child customizr.

Nathalie