Contrast fixes

scss/_alert.scss

@@ -34,6 +34,7 @@
 .alert-link {
   font-weight: $alert-link-font-weight;
   color: var(--#{$prefix}alert-link-color);
+  text-decoration: underline;
 }
 
 
@@ -69,7 +70,13 @@
     --#{$prefix}alert-color: #{color-contrast($value)};
     --#{$prefix}alert-bg: #{$value};
     --#{$prefix}alert-border-color: #{$value};
-    --#{$prefix}alert-link-color: #{shade-color(color-contrast($value), 10%)};
+    // The link deliberately keeps the alert's own text color. These alerts are
+    // solid and saturated, so `--#{$prefix}alert-color` is already at maximum
+    // contrast against the fill; any shade or tint of it can only reduce
+    // contrast. (The previous `shade-color(color-contrast($value), 10%)` shaded
+    // *white* to #e6e6e6 on the six dark alerts, making links dimmer than the
+    // surrounding text.) Emphasis comes from weight and the underline instead.
+    --#{$prefix}alert-link-color: var(--#{$prefix}alert-color);
   }
 }
 // scss-docs-end alert-modifiers