<form action="">
  <p>
    <label for="pseudo">Pseudo : </label><br>
    <input type="text" id="pseudo" class="sansBordure" /> <br>
    <label for="message">Message :</label> <br>
    <textarea name="" id="message" class="sansBordure"></textarea> <br>
    <input type="button" value='ok' />
    <input type="reset" />
  </p>
</form>

form p .sansBordure{
  border: none;
  background-color: silver;
}