Customize the header

hi all,

i want to customize the header section of my template by adding an image background (950*470px) and some buttons (here is a screencap : https://d.pr/i/qkOc ),the problem is that i can’t locate the right position to add my code (html + css).

here is my style.css : https://pastebin.com/rxxYQzqV
header.php : https://pastebin.com/aGXBdEm2

thanks for your help ! it will save my life ??