Code editor theme enhancements (#31629)

1. Fixed border-radius
2. Monaco ignores the alpha channel on the shadow color, introduce
`color-shadow-opaque`
3. Remove scrollbar color which follows
https://github.com/go-gitea/gitea/pull/29800

Before:

<img width="34" alt="Screenshot 2024-07-13 at 15 38 18"
src="https://github.com/user-attachments/assets/042d9bde-6db9-4467-a2a4-8f61ecc773eb">
<img width="35" alt="Screenshot 2024-07-13 at 15 38 31"
src="https://github.com/user-attachments/assets/04146ee0-551c-4ff2-9636-bd119b33595a">


After:

<img width="45" alt="Screenshot 2024-07-13 at 15 38 06"
src="https://github.com/user-attachments/assets/1f58fa5a-1289-4e45-83c9-18ca82a5e266">
<img width="39" alt="Screenshot 2024-07-13 at 21 16 56"
src="https://github.com/user-attachments/assets/e12ebe22-b29b-4798-9f0d-4c100f311562">

web_src/js/features/codeeditor.ts

@@ -96,7 +96,7 @@ export async function createMonaco(textarea, filename, editorOpts) {
       'input.background': getColor('--color-input-background'),
       'input.border': getColor('--color-input-border'),
       'input.foreground': getColor('--color-input-text'),
-      'scrollbar.shadow': getColor('--color-shadow'),
+      'scrollbar.shadow': getColor('--color-shadow-opaque'),
       'progressBar.background': getColor('--color-primary'),
       'focusBorder': '#0000', // prevent blue border
     },