Fix issue with #3360

See https://github.com/Jermolene/TiddlyWiki5/pull/3360#issuecomment-406400700

core/palettes/Vanilla.tid

@@ -57,8 +57,8 @@ page-background: #f4f4f4
 pre-background: #f5f5f5
 pre-border: #cccccc
 primary: #5778d8
-select-tag-background: #fff
-select-tag-foreground: #000
+select-tag-background:
+select-tag-foreground:
 sidebar-button-foreground: <<colour foreground>>
 sidebar-controls-foreground-hover: #000000
 sidebar-controls-foreground: #aaaaaa