MDL-75055 theme_boost: Darken close button colour in notification alerts

This is in order for the close button to have a good contrast against
the alert background and meet the minimum Level AA contrast ratio of 3:1

theme/boost/scss/moodle/core.scss

@@ -2678,8 +2678,15 @@ $picker-emojis-per-row: 7 !default;
 }
 
 @each $color, $value in $theme-colors {
-    .alert-#{$color} a {
-        color: darken(theme-color-level($color, $alert-color-level), 10%);
+    .alert-#{$color} {
+        a {
+            color: darken(theme-color-level($color, $alert-color-level), 10%);
+        }
+        // Darken the close button text colour inside notification alerts for better contrast.
+        .close {
+            color: darken(theme-color-level($color, $alert-color-level), 20%);
+            opacity: 0.6;
+        }
     }
 }
 .alert a {

theme/boost/style/moodle.css

@@ -12007,27 +12007,59 @@ input[disabled] {
 .alert-primary a {
   color: #041d34; }
+.alert-primary .close {
+  color: #000305;
+  opacity: 0.6; }
+
 .alert-secondary a {
   color: #525557; }
+.alert-secondary .close {
+  color: #393b3d;
+  opacity: 0.6; }
+
 .alert-success a, .environmenttable .ok a {
   color: #0c1b0b; }
+.alert-success .close, .environmenttable .ok .close {
+  color: black;
+  opacity: 0.6; }
+
 .alert-info a {
   color: #00171b; }
+.alert-info .close {
+  color: black;
+  opacity: 0.6; }
+
 .alert-warning a, .environmenttable .warn a {
   color: #573e1c; }
+.alert-warning .close, .environmenttable .warn .close {
+  color: #302310;
+  opacity: 0.6; }
+
 .alert-danger a, .environmenttable .error a {
   color: #3d0f0a; }
+.alert-danger .close, .environmenttable .error .close {
+  color: #110403;
+  opacity: 0.6; }
+
 .alert-light a {
   color: #686868; }
+.alert-light .close {
+  color: #4e4e4f;
+  opacity: 0.6; }
+
 .alert-dark a {
   color: #040505; }
+.alert-dark .close {
+  color: black;
+  opacity: 0.6; }
+
 .alert a {
   font-weight: 700; }

theme/classic/style/moodle.css

@@ -12007,27 +12007,59 @@ input[disabled] {
 .alert-primary a {
   color: #041d34; }
+.alert-primary .close {
+  color: #000305;
+  opacity: 0.6; }
+
 .alert-secondary a {
   color: #525557; }
+.alert-secondary .close {
+  color: #393b3d;
+  opacity: 0.6; }
+
 .alert-success a, .environmenttable .ok a {
   color: #0c1b0b; }
+.alert-success .close, .environmenttable .ok .close {
+  color: black;
+  opacity: 0.6; }
+
 .alert-info a {
   color: #00171b; }
+.alert-info .close {
+  color: black;
+  opacity: 0.6; }
+
 .alert-warning a, .environmenttable .warn a {
   color: #573e1c; }
+.alert-warning .close, .environmenttable .warn .close {
+  color: #302310;
+  opacity: 0.6; }
+
 .alert-danger a, .environmenttable .error a {
   color: #3d0f0a; }
+.alert-danger .close, .environmenttable .error .close {
+  color: #110403;
+  opacity: 0.6; }
+
 .alert-light a {
   color: #686868; }
+.alert-light .close {
+  color: #4e4e4f;
+  opacity: 0.6; }
+
 .alert-dark a {
   color: #040505; }
+.alert-dark .close {
+  color: black;
+  opacity: 0.6; }
+
 .alert a {
   font-weight: 700; }