WordPress custom CSS file overrided by internal CSS [closed]

First of all it should be: background-position:0 80% !important;
You said this style is applied to sections. How?
If you just add another selector, like instead using:

article{
background-position:0 80% !important;
}

use:

body article{
background-position:0 80% !important;
}

it may just work without any hacks.