Fix diagrams being hard to read with certain VSCode themes.

res/diagram.html

@@ -12,7 +12,7 @@
 	<style type="text/css">
 		body {
 			padding: 10px;
-			background-color: var(--vscode-editor-background);
+			background-color: var(--vscode-editor-selectionHighlightBackground);
 		}
 		
 		span#diagramText {
@@ -52,7 +52,7 @@
 
 		.note rect,
 		.note path {
-			fill: var(--vscode-editor-selectionHighlightBackground);
+			fill: var(--vscode-editor-background);
 			stroke: var(--vscode-editor-foreground);
 		}