Fix Cody Web Svelte Sidebar (#64320)

Cody's Web Sidebar in Svelte is not following the theme when tested
locally.

This PR fixes that. I have tested this with multiple builds both with &
without the diff.

## Test plan

Before:

![image](https://github.com/user-attachments/assets/b2c669fa-a0a7-4dc7-9440-8fca7aa69fd5)

After:
![CleanShot 2024-08-07 at 12 20
19@2x](https://github.com/user-attachments/assets/66bda613-d811-47d0-b0c5-23707e60f875)


## Changelog

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https://www.notion.so/sourcegraph/Writing-a-changelog-entry-dd997f411d524caabf0d8d38a24a878c
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Author: thenamankumar

client/web-sveltekit/src/lib/cody/CodySidebarChat.svelte

@@ -80,6 +80,7 @@ function getTelemetrySourceClient(): string {
 
 <style lang="scss">
     .chat {
+        --vscode-sideBar-background: transparent;
         --vscode-editor-background: var(--body-bg);
         --vscode-editor-foreground: var(--body-color);
         --vscode-input-background: var(--input-bg);