Adding custom header/footer

Hi There,

I’m new to HUGO and loving it so far! The only issue i’m having is with customizing themes.
I understand that I need Hugo Extended. I have that installed.

I found a theme that allows to have a custom head (through layouts/partial/custom-head.html
But when I link an external CSS it won’t wont.

I have an existing bootstrap site, and I want to use Hugo for blogging but add my header/footer of my regular site. I find it hard to accomplish. Or am I looking at the wrong themes?

Can anyone point me in the right direction?