Permalink takes on same look of thumbnail posts

[biggest fan]

So I've been trying to make a new theme for myself just to practice. It didn't go well as everything seems messed up the permalink post don't show the actual post just the same post thumbnail on the main page(with exact same styling.) On the main page the next/previous link is messed up. I'm trying to get a fixed header on scroll which doesn't seem to work. There are possibly other issues I've overlooked. After looking over the code so many times I might just be constantly overlooking it. Help would be much appreciated!

Tumblr website with the issue.