fix(stepper): incorrect border color in dark theme for header with label position (#13791)

Fixes the extra border that gets added in `labelPosition="bottom"` not being colored based on the theme.

Author: crisbeto

src/lib/stepper/_stepper-theme.scss

@@ -54,6 +54,8 @@
     border-left-color: mat-color($foreground, divider);
   }
 
+  .mat-horizontal-stepper-header::before,
+  .mat-horizontal-stepper-header::after,
   .mat-stepper-horizontal-line {
     border-top-color: mat-color($foreground, divider);
   }

src/lib/stepper/stepper.scss

@@ -40,7 +40,6 @@ $mat-stepper-line-gap: 8px !default;
 }
 
 %mat-header-horizontal-line-label-position-bottom {
-  border-top-color: rgba(0, 0, 0, 0.12);
   border-top-width: $mat-stepper-line-width;
   border-top-style: solid;
   content: '';