Improve style of notice/alert messages (#8973)

app/javascript/styles/mastodon-light/diff.scss

@@ -286,20 +286,6 @@
   }
 }
 
-.flash-message {
-  box-shadow: none;
-
-  &.notice {
-    background: rgba($success-green, 0.5);
-    color: lighten($success-green, 12%);
-  }
-
-  &.alert {
-    background: rgba($error-red, 0.5);
-    color: lighten($error-red, 12%);
-  }
-}
-
 .simple_form,
 .table-form {
   .warning {

app/javascript/styles/mastodon/forms.scss

@@ -460,9 +460,20 @@ code {
   border-radius: 4px;
   padding: 15px 10px;
   margin-bottom: 30px;
-  box-shadow: 0 0 5px rgba($base-shadow-color, 0.2);
   text-align: center;
 
+  &.notice {
+    border: 1px solid rgba($valid-value-color, 0.5);
+    background: rgba($valid-value-color, 0.25);
+    color: $valid-value-color;
+  }
+
+  &.alert {
+    border: 1px solid rgba($error-value-color, 0.5);
+    background: rgba($error-value-color, 0.25);
+    color: $error-value-color;
+  }
+
   p {
     margin-bottom: 15px;
   }