[NO GBP] Fix 516 scrollbar and background image (#88617)

Ports - ParadiseSS13/Paradise#26433
Fixes scrollbar color for Byond 516, and background position

Colorfull scrollbar
<details><summary> Videos </summary>

https://github.com/user-attachments/assets/969b4f6f-f553-4786-86c7-df11c034675b

</details>

:cl:
fix: Fixed scrollbar colors and background position in TGUI on Byond 516
/:cl:

Author: AyIong

html/statbrowser.css

@@ -3,6 +3,11 @@
 	--scrollbar-thumb: #a7a7a7;
 }
 
+html,
+body {
+	scrollbar-color: var(--scrollbar-thumb) var(--scrollbar-base);
+}
+
 body {
 	font-family: Verdana, Geneva, Tahoma, sans-serif;
 	font-size: 12px;
@@ -182,9 +187,16 @@ img {
 	margin-bottom: 1em;
 }
 
-/* Dark theme colors */
+/**
+ * MARK: Dark theme colors
+ */
+.dark:root {
+	--scrollbar-base: #151515;
+	--scrollbar-thumb: #363636;
+}
+
 body.dark {
-	background-color: #131313;
+	background-color: #151515;
 	color: #b2c4dd;
 	scrollbar-base-color: #1c1c1c;
 	scrollbar-face-color: #3b3b3b;
@@ -206,7 +218,7 @@ body.dark {
 }
 
 .dark #menu {
-	background-color: #131313;
+	background-color: #151515;
 }
 
 .dark #menu.tabs-classic .button.active {

html/statbrowser.js

@@ -705,9 +705,11 @@ function draw_verbs(cat) {
 function set_theme(which) {
 	if (which == "light") {
 		document.body.className = "";
+		document.documentElement.className = 'light';
 		set_style_sheet("browserOutput_white");
 	} else if (which == "dark") {
 		document.body.className = "dark";
+		document.documentElement.className = 'dark';
 		set_style_sheet("browserOutput");
 	}
 }

tgui/packages/tgui/layouts/Layout.jsx

@@ -10,6 +10,8 @@ import { addScrollableNode, removeScrollableNode } from '../events';
 
 export const Layout = (props) => {
   const { className, theme = 'nanotrasen', children, ...rest } = props;
+  document.documentElement.className = `theme-${theme}`;
+
   return (
     <div className={'theme-' + theme}>
       <div