"Untagged" tag uses the theme accent color (#10111)

Signed-off-by: 21pages <sunboeasy@gmail.com>

flutter/lib/models/ab_model.dart

@@ -648,6 +648,9 @@ class AbModel {
   }
 
   Color getCurrentAbTagColor(String tag) {
+    if (tag == kUntagged) {
+      return MyTheme.accent;
+    }
     int? colorValue = current.tagColors[tag];
     if (colorValue != null) {
       return Color(colorValue);