Default webview background to transparent

Webviews can now be show in places besides the editor (such as custom views). `transparent` is a better default since this lets the webiew automatically pick up the color from whatever it is shown over

src/vs/workbench/contrib/webview/browser/pre/main.js

@@ -58,7 +58,7 @@
 
 	const defaultCssRules = `
 	body {
-		background-color: var(--vscode-editor-background);
+		background-color: transparent;
 		color: var(--vscode-editor-foreground);
 		font-family: var(--vscode-font-family);
 		font-weight: var(--vscode-font-weight);