gnome-shell: Use color variable for screen shield clock

Clock/date in the unlock screen shield should use the same foreground
color as the panel button text

#72

Author: drakkar1969

common/gnome-shell/3.36/sass/_common.scss

@@ -2606,7 +2606,7 @@ StScrollBar {
 // Screen Shield
 //
 .unlock-dialog-clock {
-  color: white;
+  color: $_shell_fg_color;
   font-weight: 300;
   text-align: center;
   spacing: 24px;