MDL-83047 theme_boost: Apply background colour only on select layouts

* Apply the styling of `background-color` for `.page-header-headings`
only in the maintenance and secure layouts.

theme/boost/scss/moodle/core.scss

@@ -3127,8 +3127,11 @@ blockquote {
     border-left: 5px solid $gray-500;
 }
 
-.page-header-headings {
-    background-color: $body-bg;
+.pagelayout-maintenance,
+.pagelayout-secure {
+    .page-header-headings {
+        background-color: $body-bg;
+    }
 }
 
 /* Prevent long strings exceeding page width */

theme/boost/style/moodle.css

@@ -25953,7 +25953,8 @@ blockquote {
   border-left: 5px solid #8f959e;
 }
 
-.page-header-headings {
+.pagelayout-maintenance .page-header-headings,
+.pagelayout-secure .page-header-headings {
   background-color: #fff;
 }
 

theme/classic/style/moodle.css

@@ -25953,7 +25953,8 @@ blockquote {
   border-left: 5px solid #8f959e;
 }
 
-.page-header-headings {
+.pagelayout-maintenance .page-header-headings,
+.pagelayout-secure .page-header-headings {
   background-color: #fff;
 }