Set color-scheme property of :root element (#632)

By setting the color-scheme property of the :root element we get
matching (dark or light) scrollbars and form controls.

see: https://developer.mozilla.org/en-US/docs/Web/CSS/color-scheme

assets/js/coder.js

@@ -33,6 +33,7 @@ function setTheme(theme) {
     let inverse = theme === 'dark' ? 'light' : 'dark';
     body.classList.remove('colorscheme-' + inverse);
     body.classList.add('colorscheme-' + theme);
+    document.documentElement.style['color-scheme'] = theme;
 }
 
 function rememberTheme(theme) {