[4.0] [a11y] Modal button colour

Make sure the secondary button passes the color contrast check

### Before

### After

(Does not need npm to test)

Author: brianteeman

administrator/templates/atum/css/template-rtl.css

@@ -655,10 +655,12 @@ iframe {
   margin-right: .5rem; }
 
 .modal .btn-primary:not([href]),
-.modal .btn-success:not([href]) {
+.modal .btn-success:not([href]),
+.modal .btn-secondary:not([href]) {
   color: #fff; }
   .modal .btn-primary:not([href]):hover,
-  .modal .btn-success:not([href]):hover {
+  .modal .btn-success:not([href]):hover,
+  .modal .btn-secondary:not([href]):hover {
     color: #fff; }
 
 .modal-header {

administrator/templates/atum/css/template-rtl.min.css

@@ -655,10 +655,12 @@ iframe {
   margin-right: .5rem; }
 
 .modal .btn-primary:not([href]),
-.modal .btn-success:not([href]) {
+.modal .btn-success:not([href]),
+.modal .btn-secondary:not([href]) {
   color: #fff; }
   .modal .btn-primary:not([href]):hover,
-  .modal .btn-success:not([href]):hover {
+  .modal .btn-success:not([href]):hover,
+  .modal .btn-secondary:not([href]):hover {
     color: #fff; }
 
 .modal-header {

administrator/templates/atum/css/template.css

@@ -7,7 +7,8 @@
   }
 
   .btn-primary:not([href]),
-  .btn-success:not([href]) {
+  .btn-success:not([href]),
+  .btn-secondary:not([href]), {
     color: #fff;
 
     &:hover {