This extends my code for applying Twine tags to passage divs, and extends on it as a mechanism for CSS selection. What these macros do, in order, is add a tag to the current passage's tag attribute, remove a tag, and toggle its presence (remove it if it's there, add it if it isn't). This means that conditional application of CSS, controlled by << if >> statements, is possible.
Usage examples: <<addtag "dream">> <<toggletag "lightworld">> <<removetag "darkworld">>

These are guaranteed to work both inside passages, inside << replace >> macros, and, tantalisingly enough, inside << display >> macros. Interesting, very interesting.

If you're using the latest Twine 1.3.6 Alpha (28-1-13) , use this code:

version.extensions["toggletagMacros"]={major:1,minor:1,revision:0};macros["toggletag"]={handler:function(a,b,c){var p=e(a);var d=document.body;if(p){var t=p.getAttribute("data-tags");var i=t.indexOf(c[0]);if(b!="addtag"&&i>=0){var r=t.replace(c[0],"");p.setAttribute("data-tags",r);d.setAttribute("data-tags",r);}else{if(b!="removetag"&&i<0){var r=t+" "+c[0];p.setAttribute("data-tags",r);d.setAttribute("data-tags",r);}}}function e(f){while(f.parentNode&&!f.classList.contains("passage")){f=f.parentNode;
}if(f!=document&&f.getAttribute("data-tags")){return f;}return null;}}};macros["addtag"]=macros["toggletag"];macros["removetag"]=macros["toggletag"];

If you're using a standard version of Twine, use this code instead:

version.extensions["toggletagMacros"]={major:1,minor:1,revision:0};macros["toggletag"]={handler:function(a,b,c){var p=e(a);var d=document.body;if(p){var t=p.getAttribute("data-tags");var i=t.indexOf(c[0]);if(b!="addtag"&&i>=0){var r=t.replace(c[0],"");p.setAttribute("data-tags",r);d.setAttribute("data-tags",r);}else{if(b!="removetag"&&i<0){var r=t+" "+c[0];p.setAttribute("data-tags",r);d.setAttribute("data-tags",r);}}}else{var t=state.history[0].passage.tags;var i=t.indexOf(c[0]);if(b!="addtag"&&i>=0){t.splice(i,1);
}else{if(b!="removetag"&&i<0){t.push(c[0]);}}d.setAttribute("data-tags",t.join(" "));}function e(f){while(f.parentNode&&!f.classList.contains("passage")){f=f.parentNode;}if(f!=document&&f.getAttribute("data-tags")){return f;}return null;}}};macros["addtag"]=macros["toggletag"];macros["removetag"]=macros["toggletag"];

Version history:

1. 1.1.0 - Now adjusts tags on the body element as well.
2. 1.0.1 - Fixed not working in Jonah.
3. 1.0.0 - Initial.