Fix problem with tc-btn-invisible not correct fill color (#5288)

themes/tiddlywiki/vanilla/base.tid

@@ -483,8 +483,9 @@ button svg, button img, label svg, label img {
 	margin: 0;
 	background: none;
 	border: none;
-    	cursor: pointer;
+	cursor: pointer;
 	color: <<colour foreground>>;
+	fill: <<colour foreground>>;
 }
 
 .tc-btn-boxed {