index.css

/** Buttons */
.btn-toggler,
.btn-link {
    outline-width: 0 !important;
    background-color: transparent;
    font-family: arial, sans-serif;
}

.btn-link {
    font-size: 16px;
    border-width: 0;
    padding: 0;
    text-decoration: underline;
    cursor: pointer;
}

.btn-toggler {
    font-size: 1.5rem;
    line-height: 1;
    text-transform: uppercase;
    padding: 0.25rem 0.5rem;
    min-width: 50px;
    color: tomato;
    border: 2px solid currentColor;
}

.btn-toggler:not(:last-of-type) {
    margin-right: 0.5rem;
}

.btn-menu[aria-expanded='true'],
.btn-close[aria-expanded='false'] {
    display: none;
}

.btn-submenu span {
    font-size: 0.5em;
    line-height: 1.75;
}

/** Nav menus */
.nav-menu ul {
    display: flex;
    flex-direction: column;
    list-style: none;
    padding: 0;
}

.nav-menu a,
.nav-menu .btn-link {
    display: flex;
    box-sizing: border-box;
    padding: 0.5rem;
    width: 100%;
    text-decoration: none;
    text-transform: uppercase;
    color: tomato;
}

.nav-menu a:hover,
.nav-menu .btn-link:hover {
    background-color: rgba(0, 0, 0, 0.1);
}

/** Hide element */
.hide {
    display: none !important;
}

/** Media breakpoints */
@media (min-width: 768px) {
    .d-md-none {
        display: none;
    }

    /** Show nav menu */
    .nav-menu {
        display: flex !important;
    }

    /** Display menu items horizonatally */
    .nav-menu>ul {
        flex-direction: row;
    }
}