three.js/utils/docs/custom/three.css

:root {
    --background-color-dark: #222;
    --background-color-light: #fff;
    --border-style-dark: 1px solid #444;
    --border-style-light: 1px solid #E8E8E8;
}

/* dark */

body { 
    background-color: var(--background-color-dark) !important;
}
.navbar-container {
    background-color: var(--background-color-dark) !important;
}
.sidebar-container {
    border-right: var(--border-style-dark);
}

/* light */

body.light  { 
    background-color: var(--background-color-light) !important;
}
.light .navbar-container {
    background-color: var(--background-color-light) !important;
}
.light .sidebar-container {
    border-right: var(--border-style-light);
}
.light .sidebar {
    background-color: var(--background-color-light) !important;
}