Turns out it is (as always) a bit more complicated: The macros I've written (to start/end slideshows) change the theme:
\define slideshow()
<$button set="$:/state/sidebar" setTo="no" tooltip={{$:/language/Buttons/HideSideBar/Hint}} aria-label={{$:/language/Buttons/HideSideBar/Caption}} >
<$action-sendmessage $message="tm-close-all-tiddlers"/>
<$list filter="[tag[Slides]![sort[list]]">
<$action-navigate $to=<<currentTiddler>> />
</$list>
View Slide Show
<$action-setfield $tiddler="$:/theme" text="$:/themes/tiddlywiki/punch"/>
</$button>
\end
\define endslideshow()
<$button set="$:/state/sidebar" setTo="yes">
<$action-setfield $tiddler="$:/theme" text="$:/themes/tiddlywiki/snowwhite"/>
<$action-sendmessage $message="tm-close-all-tiddlers"/>
<$action-navigate $to="HelloThere"/>
Return to the wiki
</$button>
\end
(Does it make sense to use a <$list> construction, or should I be just changing the story? How would I do that?)
In a read-only mode (i.e. served via Dropbox), my readers are asked if they want to leave the site, and that data may not be saved. Of course, this is fine with me, but requires educating my readers.
I'd like to be able to change the theme on the fly, but not trigger the save message, which would also solve my initial goal of creating a default theme that is always the theme when the wiki opens.
Thanks,
/steve.