Convert Skeleton custom variables to sass variables

I also made some small improvement for the width when you have 3 columns that it would touch the edge. I used container sizes for this
https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_containment/Contain...
because breakpoints on viewport width don't work and therefore need the container sizes to hide elements when it's a certain width.

Before the change

After the change

robertragas → credited uber_denis → .

Automatically closed - issue fixed for 2 weeks with no activity.