https://www.garant.ru/static/www/js/theme.js?933

1const pointTheme = document.querySelector('html');
2const THEME = {
3    DARK:  'dark',
4    LIGHT: 'light',
5    AUTO:  'auto',
6};
7
8setTheme = (mode) => {
9    switch (mode) {
10        case THEME.DARK: {
11            pointTheme.setAttribute('data-bs-theme', THEME.DARK);
12            return;
13        }
14        case THEME.LIGHT: {
15            pointTheme.setAttribute('data-bs-theme', THEME.LIGHT);
16            return;
17        }
18        case THEME.AUTO:
19        default: {
20            const isDark = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches;
21
22            if(isDark) {
23                pointTheme.setAttribute('data-bs-theme', THEME.DARK);
24            } else {
25                pointTheme.setAttribute('data-bs-theme', THEME.LIGHT);
26            }
27            return;
28        }
29    }
30};
31
32initTheme = () => {
33    setTheme(THEME.AUTO);
34};
35
36initTheme();