Make codemirror use tiddler-editor-border palette color (#3819)

plugins/tiddlywiki/codemirror/styles.tid

@@ -5,7 +5,7 @@ tags: [[$:/tags/Stylesheet]]
 
 .CodeMirror {
 	height: auto;
-	border: 1px solid #ddd;
+	border: 1px solid <<colour tiddler-editor-border>>;
 	line-height: 1.5;
 	font-family: {{$:/themes/tiddlywiki/vanilla/settings/editorfontfamily}};
 	font-size: {{$:/themes/tiddlywiki/vanilla/metrics/bodyfontsize}};