Fix sidebar mode background color

Summary: It seems the theme variable we used for the background color was wrong when used in the sidebar. This breaks modals.

Reviewed By: moto-meta

Differential Revision: D55434837

fbshipit-source-id: d89183267e0ed55124e9b7c4bdf112e93bd256c5

addons/vscode/webview/vscode-styles.css

@@ -15,7 +15,7 @@ html .vscode-high-contrast {
 }
 
 .webview-view .isl-root {
-  --background: var(--vscode-sidebar-background);
+  --background: var(--vscode-sideBar-background);
   outline: 1px solid var(--vscode-panel-border);
   outline-offset: -1px;
 }