The message is clear: The Dark Arts are evil and you shouldn't use them.

If that's the message someone takes, then it applies to them. It only takes a slightly more esoteric reading to see the message as: The Dark Arts are dangerous to users and to targets, most people shouldn't use them, and only those willing to sacrifice some of their simplicity and clarity of belief can make the necessary risk/reward calculations about the unlawful knowledge. It's an infohazard warning. Or a version of the old alt.hackers self-moderation trick.

It turns out there are a _LOT_ of behaviors that are prosocial when only a few undertake them, and antisocial if everyone does them. The dream of universal behavioral rules denies diversity and specialization. The dream of universal terminal goals (with flexible or at least multiple routes toward them) may or may not be feasible, but it definitely doesn't imply shared capabilities and therefore does not imply shared behaviors.

This Schelling Fence idea still works, though, you just need to replace "universal" with "usable without harm by hobbyists and beginners".

If your website looks like this people don't need to read your content in order to tell that you're a crazy person who is out of touch with how he comes off and doesn't have basic competencies like "realize that this is terrible, hire a professional". Just scroll through without reading any of it, and with your defense against the dark arts primed and ready, tell me how likely you feel that the content is some brilliant insight into the nature of time itself. It's a real signal that credibly conveys information about how unlikely this person is to have something to say which is worth listening to. Signalling that you can't make a pretty website when you can is dishonest, and the fact that you would be hindering yourself by doing so makes it no better.

When you know what you're doing, there's nothing "dark" about looking like it.

I wouldn't call the clean design of LessWrong either involving deception or manipulation and I don't think that's what most rationalists mean when they say "dark arts". The same goes for chosing logos via a competition.  

There might be some logo designs that many people would call deceptive because they might be confused with certain existing logos but it's not what most people consider as dark arts. 

If everyone made sure their arguments looked visually pleasing, would that be sustainable? Yes, in fact the world would look more beautiful so it's totally allowed.

Here's a frequent problem with using the dark arts, they very frequently have higher order effects that hurt the user and the target in ways that are difficult to immediately foresee.

In the above proposal, there are frequently times when the most effective method of communication is to be blunt, or one argument is going to inherently be more ascetically pleasing than another. In these circumstances, if you start optimizing for making arguments pretty, then you will very likely sacrificing accuracy or effectiveness. Do this too much and your map starts to disconnect from the territory. From there it becomes easy to start taking actions that look correct according to your map, but are in fact suboptimal or outright detrimental.

To put it in practice: If everyone made sure their arguments looked visually pleasing, would that be sustainable? Yes, in fact the world would look more beautiful so it's totally allowed.

These are two different things.