Set offset that is relative to the parent element.

set the parent a position property of relative:
.parent {
  position: relative
}

This set an anchor so that all sons of this parent will set their absolute position values as an offset to this parent.

set the child element the following properties:
.child {
  position: absolute;
  top: [top]
  left: [left]
}

style tag vs. all.css vs. many custom.css

Think about:
network transport
fast vs. slow developement
managing one big file that has many sections and affects many screens