Adwaita: fix /0 on text shadow color
Bug: If the fg lightness < 50% we divide by the bg lightness no matter if it's >=0 or not.
Proposed fix: Fallback to white if the bg lightness is 0.
Edited by Bilal Elmoussaoui
Bug: If the fg lightness < 50% we divide by the bg lightness no matter if it's >=0 or not.
Proposed fix: Fallback to white if the bg lightness is 0.