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

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

## Why It's Good For The Game
Colorfull scrollbar
<details><summary> Videos </summary>


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

</details>

## Changelog

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

Author: AyIong

html/statbrowser.css

@@ -1,3 +1,13 @@
+.light:root {
+	--scrollbar-base: #f2f2f2;
+	--scrollbar-thumb: #a7a7a7;
+}
+
+html,
+body {
+	scrollbar-color: var(--scrollbar-thumb) var(--scrollbar-base);
+}
+
 body {
 	font-family: Verdana, Geneva, Tahoma, sans-serif;
 	font-size: 12px;
@@ -181,9 +191,16 @@ 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;
@@ -205,7 +222,7 @@ 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/styles/main.scss

@@ -50,7 +50,7 @@
 // NT Theme
 .Layout__content {
   background-image: url('../assets/bg-nanotrasen.svg');
-  background-size: 70%;
+  background-size: 70% 70%;
   background-position: center;
   background-repeat: no-repeat;
 }