Removed `lighten` left over from SASS to Myth conversion
Removed `lighten` left over from SASS to Myth conversion

Refs #5314
- replaced SASS's `lighten` with Myth's `color(x lightness())`

dbalders/Ghost-Admin,JohnONolan/Ghost-Admin,airycanon/Ghost-Admin,TryGhost/Ghost-Admin,JohnONolan/Ghost-Admin,airycanon/Ghost-Admin,TryGhost/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,dbalders/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin