SP (Scaleable Pixels) to REM (Relative to ɛm). In google's material design everything is
designed against a 'Scaleable Pixels' unit value.
The official value of 1SP is dp = (width in pixels * 160) / screen density.
@function e__sp-to-rem($b__sp) {
@return (round($b__sp / 0.15) / 100) * 1rem;
}