My stylesheet does not load when using express route optional parameter (/:id?)

I’m trying to create a simple express project that shows a food menu and the details of each option. My problem is that if I try to set an optional parameter for a route (/:id?) my stylesheet does not load.

My router:

router.get('/detalle/:id?', controller.details)

My controller:

const controller = {
    home: function(req,res){
        return res.render('index.ejs', {platos: platos})
    },
    details: function(req,res){
        return res.render('detalleMenu.ejs', {platos: platos})
    }
}

If I load http://localhost:3000/detalle just like that I get the css stylesheet linked to detalleMenu.ejs without a problem.
The thing is that if I try to load for example http://localhost:3000/detalle/3 I get the detalleMenu.ejs but without the stylesheet!

Any ideas why this is happening?