menu.css

body {
    background-color: white;
}
.links {
	background-color:   rgba(102, 204, 51, 0.781);
	box-shadow: 0 0 32px #0003;
    font-weight: 500;
}
.links > a {
	color: black;
	padding: .75em;
	text-align: center;
	text-decoration: none;
	transition: all .5s;
}
.links > a:hover {
	background: #ffffff06;
	color: black;
}
.links > .line {
	background: black;
	height: 1px;
}

#header {
	position: fixed;
	top: 0;
	left: 0;
	right: 0;
    width: 100%;
    height: 7%;
}
.links {
	display: grid;
	grid-template-columns: repeat(var(--items), 1fr);
	position: relative;
}
.links > .line {
	opacity: 0;
	transition: all .5s;
	position: absolute;
	bottom: 0;
	left: var(--left, calc(100% / var(--items) * (var(--index) - 1)));
	width: var(--width, calc(100% / var(--items)));
	--index: 0;
}
.links > a:hover ~ .line {
	opacity: 1;
}

.links > a:nth-of-type(1):hover ~ .line { --index: 1; }
.links > a:nth-of-type(2):hover ~ .line { --index: 2; }
.links > a:nth-of-type(3):hover ~ .line { --index: 3; }
.links > a:nth-of-type(4):hover ~ .line { --index: 4; }
.links > a:nth-of-type(5):hover ~ .line { --index: 5; }
.links > a:nth-of-type(6):hover ~ .line { --index: 6; }
.links > a:nth-of-type(7):hover ~ .line { --index: 7; }
.links > a:nth-of-type(8):hover ~ .line { --index: 8; }
.links > a:nth-of-type(9):hover ~ .line { --index: 9; }
.links > a:nth-of-type(10):hover ~ .line { --index: 10; }
.links > a:last-of-type:hover ~ .line { --index: var(--items); }