I changed the theme still I have the same problem!
So, it means issue is not $:/themes/tiddlywiki/vanilla/base
but strangely when I change the position of $:/themes/tiddlywiki/vanilla/base through
$:/tags/Stylesheet
Put it somewhere at top of the list, I see my customization work!
I don't understand what is changed in TW 5.1.20pre in last month that causes customization
through class field and Custom styles by data-tags do not work anymore!
--Mohammad