youhavetrouble.github.io/main.css

@import url('https://fonts.googleapis.com/css2?family=Open+Sans&display=swap');

:root {
    --text-color: #cccccc;
    --text-color-highlight: #ffffff;
}

html {
    font-family: 'Open Sans', sans-serif;
    color: var(--text-color);
}

body {
    display: flex;
    justify-content: start;
    align-items: start;
    color: var(--text-color);
}

nav {
    padding-block: 0.5rem;
    background-color: #232323;
    height: 100vh;
    display: flex;
    position: sticky;
    flex-direction: column;
    justify-content: center;
}

nav a {
    writing-mode: tb-rl;
    transform: rotate(-180deg);
    color: #cccccc;
    transition: color 0.2s;
    padding-block: 0.2rem;
    padding-inline: 0.5rem;
}

nav a.active {
    background-color: rgba(255,255,255, 0.1);
    color: var(--text-color-highlight);
}

nav a.active::after {
    opacity: 1;
    background-color: var(--text-color-highlight);
}

nav a::after {
    content: '';
    position: absolute;
    top: 0;
    left: 0.1rem;
    width: 1px;
    height: 100%;
    background-color: var(--text-color);
    opacity: 0;
    transition: opacity 0.2s;
}

nav a:hover {
    color: white;
}

nav a:hover::after {
    height: 100%;
    color: #cccccc;
    opacity: 1;
}