Layering Logo over slider on homepage

Here is the site: https://somethingnewentertainment.com/new/

I don’t really know where to begin to change the position of the slider and the logo in the header. In the theme, the style.css has all the code. Would I enter the custom line in the header.php to override what is in the css? Thanks for any help, I’m new to customizing but want to learn.