[ui] Fix dark mode scrollbars (#26033)

## Summary & Motivation

Fix scrollbars in dark mode.

- Use `dark` instead of `'dark'` as the color scheme value. This fixes
the main issue, where we're rendering light-mode browser controls. I'm
not sure if this has been broken since we shipped dark mode, or if
something recently changed. Either way, sorry for leaving it broken for
so long.
- Style the scrollbar to use our themed colors, instead of the
(mismatching) defaults.

<img width="1098" alt="Screenshot 2024-11-20 at 08 31 03"
src="https://github.com/user-attachments/assets/638d77b9-5add-4cc2-a2ba-0de7c2c68b40">

<img width="1098" alt="Screenshot 2024-11-20 at 08 31 18"
src="https://github.com/user-attachments/assets/f062f452-0e97-42db-9311-07ba0017626b">


## How I Tested These Changes

Change MacOS settings to show scrollbars in all cases. Load an asset
detail page, shrink the viewport to force scrollbars to appear.

Verify that they look correct (including themed colors) in both dark
mode and light mode.

## Changelog

[ui] Fix scrollbars in dark mode.

js_modules/dagster-ui/packages/ui-components/src/theme/darkThemeColors.tsx

@@ -1,7 +1,7 @@
 import {css} from 'styled-components';
 
 export const darkThemeColors = css`
-  --browser-color-scheme: 'dark';
+  --browser-color-scheme: dark;
   --color-keyline-default: var(--color-translucent-gray15);
   --color-link-default: var(--color-core-blue200);
   --color-link-hover: var(--color-core-blue400);

js_modules/dagster-ui/packages/ui-core/src/app/GlobalStyleProvider.tsx

@@ -26,6 +26,7 @@ const GlobalStyle = createGlobalStyle`
     color-scheme: ${Colors.browserColorScheme()};
     background-color: ${Colors.backgroundDefault()};
     color: ${Colors.textDefault()};
+    scrollbar-color: ${Colors.accentGrayHover()} ${Colors.backgroundDefault()};
     width: 100vw;
     height: 100vh;
     overflow: hidden;