I have an issue with creating a navbar (the text isn’t in the exact middle of it, and also i don’t know how to make the whole “button” change its background color instead of just the text) and making the first paraghraph after an h2 header to be bigger.
Sourse code: https://glitch.com/edit/#!/invented-cuboid-cork
Live site: https://invented-cuboid-cork.glitch.me
Open to any assessment
Well done! 
You could remove height and line-height from the nav ul li and add the following to nav ul li a:
display: inline-block;
width: 100%;
line-height: 3;
This expands the a to the full width of the li and centers it vertically.
section:first-child(p) would select all sections if they are also the first-child. (p) is invalid syntax.
What you want to do is targeting the p directly after the h2. You can do that with the “Adjacent sibling combinator”: section h2 + p
This selects the p only if it’s the next element after an h2 (and all inside a section).
Another thing that could be improved is a bigger difference between visited and unvisited links.
I hope that helps. 
Have a nice weekend,
Michael