Hi there ph-xyz
When you go to the Controlpanel > Plugins > Themes you'll see the themes.
The base theme is the Vanilla theme. If you click on it, you'll see it contains
$:/themes/tiddlywiki/vanilla/base
which is the base stylesheet.
The standard distribution also shows, by default, this stylesheet:
$:/themes/tiddlywiki/snowwhite/base
All stylesheets are tagged $:/tags/Stylesheet (and they have type: text/css unless they also contain WikiText in which case they have type text/vnd.tiddlywiki ) and you can click this tag to get a dropdown showing all other tiddlers tagged such, i.e all other stylesheets. You can also rearrange them in this little dropdown via drag'n drop to set the cascade order.
<:-)