LIFO is the way browser parses CSS properties..If you are using Sass declare a variable called as
"$header-background: red;"
use it instead of directly assigning values like red or blue. When you want to override just reassign the value to
"$header-background:blue"
then
background-color:$header-background;
it should smoothly override. Using "!important" is not always the right choice..Its just a hotfix