Basic CSS for hamburger menu

/* Hamburger menu button */
.menu-toggle {
  display: none;
  padding: 10px 15px;
  cursor: pointer;
}

/* Hamburger menu icon */
.menu-toggle:before {
  content: "";
  display: block;
  width: 20px;
  height: 2px;
  background-color: #333;
  transition: all 0.3s;
}

/* Transform hamburger menu icon into an X */
.menu-toggle.open:before {
  transform: rotate(45deg);
}
.menu-toggle.open:after {
  transform: rotate(-45deg);
}
.menu-toggle.open:before,
.menu-toggle.open:after {
  position: absolute;
  top: 9px;
  right: 15px;
  content: "";
  display: block;
  width: 20px;
  height: 2px;
  background-color: #333;
}

/* Hide main navigation on mobile */
.main-navigation {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  text-align: center;
  background-color: #f2f2f2;
  height: 100vh;
  width: 100%;
  position: fixed;
  top: 0;
  left: 0;
  transition: all 0.3s;
  transform: translateX(-100%);
}

/* Show main navigation on click */
.menu-toggle.open + .main-navigation {
  transform: translateX(0);
}

/* Style navigation links */
.main-navigation a {
  display: block;
  padding: 20px;
  color: #333;
  text-decoration: none;
  transition: all 0.3s;
}
.main-navigation a:hover {
  background-color: #333;
  color: #f2f2f2;
}

This CSS code styles a hamburger menu button, transforms the hamburger menu icon into an X when the menu is open, and hides the main navigation on mobile, showing it on click. You can modify the styles to fit your needs and design preferences.