/*Menu position for mobile site*/
@media screen and (max-width: 768px) {
.main-navigation{
position: relative;
top: 30px;
}

button.menu-toggle {
float: none !important;
margin:auto}
}

Could you please advise how should I center the menu button? (I mean only the button, I don’t want to center the text)
Thanks in advance & Best Regards,
Marta

To center the menu button, but not the menu items themselves, you can use the following CSS code:

@media screen and (max-width: 767px) {
  #site-navigation {
    text-align: center;
  }

  #site-navigation ul {
    text-align: left;
  }
}