Add modal element

themes/ssp/default/includes/modal.php

+<!-- Modal -->
+<div class="modal fade" id="themeModal" tabindex="-1" role="dialog" aria-labelledby="themeModalLabel"
+     aria-hidden="true">
+    <div class="modal-dialog" role="document">
+        <div class="modal-content">
+            <div class="modal-header">
+                <h5 class="modal-title" id="themeModalLabel">Modal title</h5>
+                <button type="button" class="close" data-dismiss="modal" aria-label="Close">
+                    <span aria-hidden="true">&times;</span>
+                </button>
+            </div>
+            <div class="modal-body">
+                ...
+            </div>
+            <div class="modal-footer">
+                <button type="button" class="btn btn-secondary" data-dismiss="modal">Ok</button>
+            </div>
+        </div>
+    </div>
+</div>
\ No newline at end of file