Fix for alert-info modal in jackett

The .alert-info didn't have a background because the `background-color` property should've been `background` instead

CSS/themes/jackett/jackett-base.css

@@ -215,6 +215,6 @@ fieldset[disabled] .form-control {
 
 .alert-info {
     color: #ffffff;
-    background-color: var(--modal-bg-color);
+    background: var(--modal-bg-color);
     border-color: transparent;
-}
+}