.login {
  max-width: 300px;
  max-height: 300px;
  position: fixed;
  top: 50%;
  left: 50%;
  padding: 10px;
  margin-top: -150px;
  margin-left: -150px;
  vertical-align: middle;
  text-align: center;

  a {
    color: #333;
  }

  input {
    width: 100%;
    line-height: 32px;
    text-align: center;
    padding: 5px 10px;
    margin-bottom: 10px;
    background: #fff;
    border-radius: 2px;
    border: 1px solid #ccc;
    box-shadow: none;
  }

  button {
    color: #fff;
    font-size: 14px;
    background: #8abc57;
    border: 1px solid #5e902b;
    padding: 5px 10px;
    border-radius: 2px;
    width: 100%;
    line-height: 32px;
    box-shadow: inset 0 0 2px rgba(#fff, .4);
    margin-bottom: 10px;
    transition: background .3s ease;

    &:hover {
      background: lighten(#8abc57, 5%);
      box-shadow: 0 1px 2px rgba(#000, .3);
    }
  }
}

.login {
  max-width: 300px;
  max-height: 300px;
  position: fixed;
  top: 50%;
  left: 50%;
  padding: 10px;
  margin-top: -150px;
  margin-left: -150px;
  vertical-align: middle;
  text-align: center;

  a {
    color: #333;
  }

  input {
    width: 100%;
    line-height: 32px;
    text-align: center;
    padding: 5px 10px;
    margin-bottom: 10px;
    background: #fff;
    border-radius: 2px;
    border: 1px solid #ccc;
    box-shadow: none;
  }

  button {
    color: #fff;
    font-size: 14px;
    background: #8abc57;
    border: 1px solid #5e902b;
    padding: 5px 10px;
    border-radius: 2px;
    width: 100%;
    line-height: 32px;
    box-shadow: inset 0 0 2px rgba(#fff, .4);
    margin-bottom: 10px;
    transition: background .3s ease;
    height: auto;

    &:hover {
      background: lighten(#8abc57, 5%);
      box-shadow: 0 1px 2px rgba(#000, .3);
    }
  }
}

Fix height of login button
Fix height of login button
