This requires my code for applying 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.

  version.extensions['toggletag'] = {major:1, minor:0, revision:0};
  macros['toggletag'] = {
    handler: function (a, b, c) {
		var p = e(a);
		if (p) {
		  var t = p.getAttribute("tags");
		  var i = t.indexOf(c[0]);
		  if (b!="addtag" && i>=0) {
		    p.setAttribute("tags",t.replace(c[0],""));
		  }
		  else if (b!="removetag" && i<0) p.setAttribute("tags",t+" "+c[0]);
		}
		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]); 
		  }
		}
		function e(f) {
		  while(f.parentNode && f.className != "passage") { f = f.parentNode; }
		  if (f.getAttribute("tags")) return f;
		  return null;
		}
    }
  }
macros['addtag'] = macros['toggletag'];
macros['removetag'] = macros['toggletag'];