diff --git a/application/views/backend/header.php b/application/views/backend/header.php
index fa7b8b5f..8d3dc30f 100755
--- a/application/views/backend/header.php
+++ b/application/views/backend/header.php
@@ -46,7 +46,7 @@
 
     <div id="header-menu" class="collapse navbar-collapse flex-row-reverse">
         <ul class="navbar-nav">
-            <?php $hidden = ($privileges[PRIV_APPOINTMENTS]['view'] == TRUE) ? '' : 'hidden' ?>
+            <?php $hidden = ($privileges[PRIV_APPOINTMENTS]['view'] == TRUE) ? '' : 'd-none' ?>
             <?php $active = ($active_menu == PRIV_APPOINTMENTS) ? 'active' : '' ?>
             <li class="nav-item <?= $active . $hidden ?>">
                 <a href="<?= site_url('backend') ?>" class="nav-link"
@@ -55,7 +55,7 @@
                 </a>
             </li>
 
-            <?php $hidden = ($privileges[PRIV_CUSTOMERS]['view'] == TRUE) ? '' : 'hidden' ?>
+            <?php $hidden = ($privileges[PRIV_CUSTOMERS]['view'] == TRUE) ? '' : 'd-none' ?>
             <?php $active = ($active_menu == PRIV_CUSTOMERS) ? 'active' : '' ?>
             <li class="nav-item <?= $active . $hidden ?>">
                 <a href="<?= site_url('backend/customers') ?>" class="nav-link"
@@ -64,7 +64,7 @@
                 </a>
             </li>
 
-            <?php $hidden = ($privileges[PRIV_SERVICES]['view'] == TRUE) ? '' : 'hidden' ?>
+            <?php $hidden = ($privileges[PRIV_SERVICES]['view'] == TRUE) ? '' : 'd-none' ?>
             <?php $active = ($active_menu == PRIV_SERVICES) ? 'active' : '' ?>
             <li class="nav-item <?= $active . $hidden ?>">
                 <a href="<?= site_url('backend/services') ?>" class="nav-link"
@@ -73,7 +73,7 @@
                 </a>
             </li>
 
-            <?php $hidden = ($privileges[PRIV_USERS]['view'] ==  TRUE) ? '' : 'hidden' ?>
+            <?php $hidden = ($privileges[PRIV_USERS]['view'] ==  TRUE) ? '' : 'd-none' ?>
             <?php $active = ($active_menu == PRIV_USERS) ? 'active' : '' ?>
             <li class="nav-item <?= $active . $hidden ?>">
                 <a href="<?= site_url('backend/users') ?>" class="nav-link"
@@ -83,7 +83,7 @@
             </li>
 
             <?php $hidden = ($privileges[PRIV_SYSTEM_SETTINGS]['view'] == TRUE
-                    || $privileges[PRIV_USER_SETTINGS]['view'] == TRUE) ? '' : 'hidden' ?>
+                    || $privileges[PRIV_USER_SETTINGS]['view'] == TRUE) ? '' : 'd-none' ?>
             <?php $active = ($active_menu == PRIV_SYSTEM_SETTINGS) ? 'active' : '' ?>
             <li class="nav-item <?= $active . $hidden ?>">
                 <a href="<?= site_url('backend/settings') ?>" class="nav-link"
diff --git a/application/views/general/installation.php b/application/views/general/installation.php
index 7fb6db08..f7c21132 100755
--- a/application/views/general/installation.php
+++ b/application/views/general/installation.php
@@ -77,7 +77,7 @@
             </p>
         </div>
 
-        <div class="alert hidden"></div>
+        <div class="alert d-none"></div>
 
         <div class="row">
             <div class="admin-settings col-xs-12 col-sm-5">
diff --git a/application/views/user/forgot_password.php b/application/views/user/forgot_password.php
index 4089ebc5..ce5baf2e 100644
--- a/application/views/user/forgot_password.php
+++ b/application/views/user/forgot_password.php
@@ -67,7 +67,7 @@
         <h2><?= lang('forgot_your_password') ?></h2>
         <p><?= lang('type_username_and_email_for_new_password') ?></p>
         <hr>
-        <div class="alert hidden"></div>
+        <div class="alert d-none"></div>
         <form>
             <div class="form-group">
                 <label for="username"><?= lang('username') ?></label>
diff --git a/application/views/user/login.php b/application/views/user/login.php
index 24eea16a..9434815e 100644
--- a/application/views/user/login.php
+++ b/application/views/user/login.php
@@ -66,7 +66,7 @@
         <h2><?= lang('backend_section') ?></h2>
         <p><?= lang('you_need_to_login') ?></p>
         <hr>
-        <div class="alert hidden"></div>
+        <div class="alert d-none"></div>
         <form id="login-form">
             <div class="form-group">
                 <label for="username"><?= lang('username') ?></label>
diff --git a/assets/js/backend_calendar_extra_periods_modal.js b/assets/js/backend_calendar_extra_periods_modal.js
index fb1b2928..eb89f0be 100644
--- a/assets/js/backend_calendar_extra_periods_modal.js
+++ b/assets/js/backend_calendar_extra_periods_modal.js
@@ -62,11 +62,11 @@ window.BackendCalendarExtraPeriodsModal = window.BackendCalendarExtraPeriodsModa
                 $dialog.find('.modal-message')
                     .text(EALang.extra_period_saved)
                     .addClass('alert-success')
-                    .removeClass('alert-danger hidden');
+                    .removeClass('alert-danger d-none');
 
                 // Close the modal dialog and refresh the calendar appointments after one second.
                 setTimeout(function () {
-                    $dialog.find('.alert').addClass('hidden');
+                    $dialog.find('.alert').addClass('d-none');
                     $dialog.modal('hide');
 
                     var providerId = $('#extra-provider').val();
@@ -93,7 +93,7 @@ window.BackendCalendarExtraPeriodsModal = window.BackendCalendarExtraPeriodsModa
                     'the operation could not complete due to server communication errors.');
 
                 $dialog.find('.modal-message').txt(EALang.service_communication_error);
-                $dialog.find('.modal-message').addClass('alert-danger').removeClass('hidden');
+                $dialog.find('.modal-message').addClass('alert-danger').removeClass('d-none');
             };
 
             BackendCalendarApi.saveExtraPeriod(extra, successCallback, errorCallback);
diff --git a/assets/js/backend_calendar_unavailabilities_modal.js b/assets/js/backend_calendar_unavailabilities_modal.js
index f565d120..47307b77 100755
--- a/assets/js/backend_calendar_unavailabilities_modal.js
+++ b/assets/js/backend_calendar_unavailabilities_modal.js
@@ -39,7 +39,7 @@ window.BackendCalendarUnavailabilitiesModal = window.BackendCalendarUnavailabili
                 $dialog.find('.modal-message')
                     .text(EALang.start_date_before_end_error)
                     .addClass('alert-danger')
-                    .removeClass('hidden');
+                    .removeClass('d-none');
 
                 $dialog.find('#unavailable-start, #unavailable-end').closest('.form-group').addClass('has-error');
                 return;
@@ -63,7 +63,7 @@ window.BackendCalendarUnavailabilitiesModal = window.BackendCalendarUnavailabili
                 Backend.displayNotification(EALang.unavailable_saved);
 
                 // Close the modal dialog and refresh the calendar appointments.
-                $dialog.find('.alert').addClass('hidden');
+                $dialog.find('.alert').addClass('d-none');
                 $dialog.modal('hide');
                 $('#select-filter-item').trigger('change');
             };
@@ -73,7 +73,7 @@ window.BackendCalendarUnavailabilitiesModal = window.BackendCalendarUnavailabili
                     'the operation could not complete due to server communication errors.');
 
                 $dialog.find('.modal-message').txt(EALang.service_communication_error);
-                $dialog.find('.modal-message').addClass('alert-danger').removeClass('hidden');
+                $dialog.find('.modal-message').addClass('alert-danger').removeClass('d-none');
             };
 
             BackendCalendarApi.saveUnavailable(unavailable, successCallback, errorCallback);
diff --git a/assets/js/forgot_password.js b/assets/js/forgot_password.js
index 8af8d828..1f852b0c 100644
--- a/assets/js/forgot_password.js
+++ b/assets/js/forgot_password.js
@@ -22,12 +22,12 @@ $(function () {
 
         var $alert = $('.alert');
 
-        $alert.addClass('hidden');
+        $alert.addClass('d-none');
         $('#get-new-password').prop('disabled', true);
 
         $.post(url, data)
             .done(function (response) {
-                $alert.removeClass('hidden alert-danger alert-success');
+                $alert.removeClass('d-none alert-danger alert-success');
                 $('#get-new-password').prop('disabled', false);
                 if (response === GlobalVariables.AJAX_SUCCESS) {
                     $alert.addClass('alert-success');
diff --git a/assets/js/login.js b/assets/js/login.js
index 6b38a9c4..b5cf2923 100644
--- a/assets/js/login.js
+++ b/assets/js/login.js
@@ -22,7 +22,7 @@ $(function () {
 
         var $alert = $('.alert');
 
-        $alert.addClass('hidden');
+        $alert.addClass('d-none');
 
         $.post(url, data)
             .done(function (response) {
@@ -31,7 +31,7 @@ $(function () {
                 } else {
                     $alert.text(EALang['login_failed']);
                     $alert
-                        .removeClass('hidden alert-danger alert-success')
+                        .removeClass('d-none alert-danger alert-success')
                         .addClass('alert-danger');
                 }
             })
diff --git a/assets/js/working_plan.js b/assets/js/working_plan.js
index ee2ce22d..66fcf5ed 100755
--- a/assets/js/working_plan.js
+++ b/assets/js/working_plan.js
@@ -241,7 +241,7 @@
                                 }),
                                 $('<button/>', {
                                     'type': 'button',
-                                    'class': 'btn btn-default btn-sm save-extra hidden',
+                                    'class': 'btn btn-default btn-sm save-extra d-none',
                                     'title': EALang.save,
                                     'html': [
                                         $('<span/>', {
@@ -251,7 +251,7 @@
                                 }),
                                 $('<button/>', {
                                     'type': 'button',
-                                    'class': 'btn btn-default btn-sm cancel-extra hidden',
+                                    'class': 'btn btn-default btn-sm cancel-extra d-none',
                                     'title': EALang.cancel,
                                     'html': [
                                         $('<span/>', {
@@ -600,7 +600,7 @@
                             }),
                             $('<button/>', {
                                 'type': 'button',
-                                'class': 'btn btn-default btn-sm save-extra hidden',
+                                'class': 'btn btn-default btn-sm save-extra d-none',
                                 'title': EALang.save,
                                 'html': [
                                     $('<span/>', {
@@ -610,7 +610,7 @@
                             }),
                             $('<button/>', {
                                 'type': 'button',
-                                'class': 'btn btn-default btn-sm cancel-extra hidden',
+                                'class': 'btn btn-default btn-sm cancel-extra d-none',
                                 'title': EALang.cancel,
                                 'html': [
                                     $('<span/>', {
@@ -699,8 +699,8 @@
 
             // Show save - cancel buttons.
             var $tr = $(this).closest('tr');
-            $tr.find('.edit-extra, .delete-extra').addClass('hidden');
-            $tr.find('.save-extra, .cancel-extra').removeClass('hidden');
+            $tr.find('.edit-extra, .delete-extra').addClass('d-none');
+            $tr.find('.save-extra, .cancel-extra').removeClass('d-none');
             $tr.find('select,input:text').addClass('form-control input-sm')
 
             $('.add-extra-periods').prop('disabled', true);
@@ -729,8 +729,8 @@
             $modifiedRow.find('.cancel-editable').trigger('click');
             this.enableCancel = false;
 
-            $(element).closest('table').find('.edit-extra, .delete-extra').removeClass('hidden');
-            $modifiedRow.find('.save-extra, .cancel-extra').addClass('hidden');
+            $(element).closest('table').find('.edit-extra, .delete-extra').removeClass('d-none');
+            $modifiedRow.find('.save-extra, .cancel-extra').addClass('d-none');
             $('.add-extra-periods').prop('disabled', false);
         }.bind(this));
 
@@ -756,8 +756,8 @@
             $modifiedRow.find('.editable .submit-editable').trigger('click');
             this.enableSubmit = false;
 
-            $modifiedRow.find('.save-extra, .cancel-extra').addClass('hidden');
-            $(element).closest('table').find('.edit-extra, .delete-extra').removeClass('hidden');
+            $modifiedRow.find('.save-extra, .cancel-extra').addClass('d-none');
+            $(element).closest('table').find('.edit-extra, .delete-extra').removeClass('d-none');
             $('.add-extra-periods').prop('disabled', false);
         }.bind(this));
     };