One expect to have the theme stylesheet tiddlers at the very begining or list of tiddlers tagged with $:/tags/Stylesheet.
This way one can override theme stylesheets and have customized css for his/her wiki!
Strangely in TW5.1.20pre, it seems theme cssa are not at top of the list and they override other plugin or user defined CSSs.
Even if you look at the
https://tiddlywiki.com/prerelease/#%24%3A%2Fcore%2Fui%2FControlPanel%2FStylesheets
You see snowwhite and vanilla base tiddlers are at position 5 and 6.
So, it seems TW 5.1.20pre has some changes that cause theme css tiddlers appear after some user/plugin css tiddlers and that
overrides user customization.
--Mohammad
For me it is critical, as some of my customization (Stylesheets) do not work!