Agregar las clases y solo alternar la clase show en la clase nav con el evento click en x elemento, los colores se cambian desde la variable

_navbar.scss

.nav {
  display: flex;
  font-size: 18px;
  width: 100%;
  height: 75px;
  justify-content: space-between;
  align-items: center;

  &-title {
    color: #fff;
    font-size: 35px;
    padding: 0 80;
    font-weight: bold;
  }

  &-list {
    display: flex;
    list-style: none;
    margin-right: 30px;
    z-index: 10;
  }

  &-list li {
    border-radius: 3px;
    margin: 0 5px;
  }
  &-list li:hover,
  li:active {
    //background-color: rgba(0, 0, 0, .3);
    text-shadow: 2px 2px 5px #999;
    transition: 0.4s;
  }

  &-list-item {
    display: block;
    padding: 7px 13px;
    text-decoration: none;
    font-size: 18px;
    color: #fff;
    text-transform: uppercase;
  }
}
#btn-nav {
  margin-right: 30px;
  font-size: 25px;
  color: #fff;
  cursor: pointer;
  display: none;
}

ul.show {
  top: 65px;
}

@media screen and (max-width: 952px) {
  .nav-title {
    font-size: 30px;
    padding-left: 35px;
  }
  .nav-list-item {
    font-size: 16px;
  }
}

@media screen and (max-width: 858px) {
  .nav {
    height: 65px;
  }
  #btn-nav {
    display: inline-flex;
  }
  .nav-list {
    position: fixed;
    flex-direction: column;
    width: 100%;
    height: calc(100% - 65px);
    background-color: $secondary;
    top: 100vh;
    text-align: center;
    transition: all 0.4s;
  }
  .nav-list li {
    margin: 30px 0 0;
    line-height: 30px;
    & :hover {
      background: none;
    }
  }
  .nav-list-item {
    font-size: 20px;
    font-weight: bold;
  }
  .nav-title {
    font-size: 25px;
    padding-left: 35px;
  }
}

Navbar Responsive with SCSS

import { useState } from "react";

const Header = () => {
  const [showMenu, setShowMenu] = useState(false);
  return (
    <div>
      <nav className="nav">
        <p className="nav-title">PixaHi</p>
        <ul className={`nav-list ${showMenu ? "show" : ""}`}>
          <li>
            <a className="nav-list-item" href="/">
              Fotos
            </a>
          </li>
          <li>
            <a className="nav-list-item" href="/">
              Ilustraciones
            </a>
          </li>
        </ul>
        <div id="btn-nav" onClick={() => setShowMenu(!showMenu ? true : false)}>
          <i className="fas fa-bars"></i>
        </div>
      </nav>
    </div>
  );
};

export default Header;

styles.scss

$primary: #5039E7;
$secondary: #7A58FF;
$tither: #A178FF;
$for: #C798FF;
$success: #5039E7;

*, *:after,  *:before{
  box-sizing: border-box;
  padding: 0;
  margin: 0;
  outline: none;
  box-sizing: border-box;
}

body{
  background-color: $primary;
  color: #fff;
  font-size: 14px;
  font-family: 'Roboto', sans-serif;
}

@import './_navbar';