WorkflowyStylableTags_big_old_version.js

// ==UserScript==
// @name           WorkflowyStylableTags
// @description    Gives each tag it's own css style, so you can style them with Stylish. I use Blank Canvas to manage my userscripts in Chrome.
// @author         Nigel Thorne
// @include        http*://*workflowy.com/*
// @version        1.0
// ==/UserScript==

String.prototype.endsWith = function(suffix) {
    return this.indexOf(suffix, this.length - suffix.length) !== -1;
};

var customClasses = function(index, old){
	var classes = old.split(" ");
	var custom = [];
	for( i = 0; i < classes.length; i++){
		if(classes[i].endsWith("-row")){ custom.push(classes[i]);};
		if(classes[i].endsWith("-span")){ custom.push(classes[i]);};
		if(classes[i].endsWith("-proj")){ custom.push(classes[i]);};
	};
	return custom.join(" ");
}

var StylableTagsCounter = 1;
setInterval(function(){
    StylableTagsCounter ++;
    if( StylableTagsCounter >= 3){
        $('.name').removeClass(customClasses);
        $('.content').removeClass(customClasses);
        $('.project').removeClass(customClasses);
        $('.pageContainer').removeClass(customClasses);
        StylableTagsCounter = 0;
    }
	$('span > .contentTagText').map( function(){
		var x = $(this).text();
		$(this).parent('.contentTag').addClass(x).parent().addClass(x+"-span").parent().addClass(x+"-row").parent().addClass(x+"-proj");}
	);
},1000);

WorkflowyStylableTags_slim_version.js

// ==UserScript==
// @name           WorkflowyStylableTags
// @description    Gives each tag it's own css style, so you can style them with Stylish. I use Blank Canvas to manage my userscripts in Chrome.
// @author         Nigel Thorne and LukeMT
// @include        http*://*workflowy.com/*
// @version        1.1
// ==/UserScript==

String.prototype.endsWith = function(suffix) {
    return this.indexOf(suffix, this.length - suffix.length) !== -1;
};

var customClasses = function(index, old){
	var classes = old.split(" ");
	var custom = [];
	for( i = 0; i < classes.length; i++){
		if(classes[i].endsWith("-proj")){ custom.push(classes[i]);};
	};
	return custom.join(" ");
}

var StylableTagsCounter = 1;
setInterval(function(){
    StylableTagsCounter ++;
    if( StylableTagsCounter >= 3){
        $('.project').removeClass(customClasses);
        $('.pageContainer').removeClass(customClasses);
        StylableTagsCounter = 0;
    }
	$('span > .contentTagText').map( function(){
		var x = $(this).text();
		$(this).parent('.contentTag').parent().parent().parent().addClass(x+"-proj");}
	);
},1000);

Your selector for pageContainer selects on class, but pageContainer is an id.
Here's a fix: https://gist.github.com/Mqrius/2012e139b1fc4c3ccaf2a5fe0a67bfec/revisions
(Can't do pull requests on gists)
(Note that I haven't fixed the big old version. Fix is trivial though.)