Campos formulario error css

Típico campo de formulario.

<div class="form-group">
  <label asp-for="Name">Your name:</label>
  <input asp-for="Name" class="form-control">
  <span asp-validation-for="Name" class="small"></span>
</div>

Añadir en el archivo main.css, modificar al gusto del consumidor.

.field-validation-error    {color: #dc3545;}
.field-validation-valid    { display: none;}
.input-validation-error    { border: 1px solid #dc3545; }
.validation-summary-valid  { display: none;}
.validation-summary-errors { color: #dc3545; }
.validation-summary-errors ul {
    list-style: none;
    padding: 0.5rem;
    border: 1px solid #dc3545;
    background-color: #ffebee;
}