diff --git a/static/themes/default/css/main.css b/static/themes/default/css/main.css
index ddedfa723..9559e1fa7 100644
--- a/static/themes/default/css/main.css
+++ b/static/themes/default/css/main.css
@@ -8,6 +8,12 @@
     --progress-color: #3498db; /* Blue */
     --freedombox-form-disabled-bg-color: #e9ecef; /* Grey */
     --freedombox-navbar-color: white;
+
+    /* Missing variables in Bootstrap 5.2 copied from Bootstrap 5.3 */
+    --bs-secondary-bg: #e9ecef;
+    --bs-form-invalid-color: #dc3545;
+    --bs-form-invalid-border-color: #dc3545;
+    --bs-secondary-border-subtle: #c4c8cb;
 }
 
 @font-face {