Here's a UX opinion I'm ready to defend: when in dark mode, the theme toggle should show a sun. When in light mode, it should show a moon.

This might sound obvious to some, but I've seen countless implementations get this backwards—and after carefully considering this during my work on docs.deno.com and fresh.deno.dev/docs, I'm convinced this approach should become the universal standard.

The Principle: Buttons Indicate Actions, Not States

Let me break down the fundamental UX principle at play here:

A button's icon should communicate what will happen when you interact with it, not describe the current state of the application.

Think about it this way: when you're looking at a dark-themed page, you already know it's dark. The entire interface—the background, the text colors, the overall ambiance—is screaming "dark mode" at you. You don't need a moon icon to confirm what your eyes can plainly see.

What you do need is guidance on what the button will do. And in dark mode, clicking the theme toggle will take you to light mode. Hence: show a sun.

The inverse is equally true: in light mode, the page is obviously bright and light-colored. You don't need a sun icon to tell you that. But you do want to know that clicking this button will take you somewhere darker and easier on the eyes at night. Hence: show a moon.

The Anti-Pattern: Icons as State Indicators

The opposite approach—showing a moon when in dark mode and a sun when in light mode—treats the toggle as a status indicator rather than an action trigger. This is problematic for several reasons:

1. Redundant Information

If I'm staring at a white page with black text, I don't need a sun icon to confirm I'm in light mode. That's like having a "You are currently standing" sign next to an elevator button.

2. Cognitive Friction

When the icon shows the current state, users must perform an extra mental step: "I see a moon, which means I'm in dark mode, which means clicking this will take me to... light mode?" That's unnecessary cognitive load.

3. Inconsistent with Standard Button Behavior

Consider every other button in your interface:

A play button doesn't show the "playing" state—it shows what happens when you click it
A mute button shows a speaker with a line through it to indicate "click to mute," not "audio is currently unmuted"
A hamburger menu doesn't change based on whether menus exist—it indicates "click to open menu"

Why should theme toggles be any different?

My Implementation Journey with Deno

When I worked on adding dark/light mode support to Deno's documentation sites, I spent considerable time thinking through this problem. These sites serve developers—people who appreciate thoughtful, intuitive interfaces.

The implementation I settled on uses a clever SVG animation that morphs between sun and moon icons. The key CSS that makes this work correctly:

/* In dark mode: show the sun (to switch to light) */
.dark .sun-and-moon > .sun {
  transform: scale(1);
}
.dark .sun-and-moon > .sun-beams {
  opacity: 1;
}
.dark .sun-and-moon > .moon > circle {
  transform: translateX(-7px); /* Hide the moon mask */
}

/* In light mode: show the moon (to switch to dark) */
.light .sun-and-moon > .sun {
  transform: scale(0.5);
}
.light .sun-and-moon > .sun-beams {
  opacity: 0;
}
.light .sun-and-moon > .moon > circle {
  transform: translateX(0); /* Show the moon crescent */
}

The sun morphs into a moon (and vice versa) with smooth transitions, making it crystal clear what the button does without requiring any additional thought from the user.

A Call to Action for Developers

I genuinely hope to see this become the standard approach for theme toggles across the web. If you're implementing a dark/light mode toggle, consider these guidelines:

The Standard

Current Theme	Toggle Icon	Action on Click
Dark Mode	☀️ Sun	Switch to Light
Light Mode	🌙 Moon	Switch to Dark

Additional Best Practices

Use smooth transitions — A morphing animation between sun and moon reinforces the connection between the icon and the action.
Include proper ARIA labels — Use aria-label to explicitly state what the button does: "Switch to light mode" or "Switch to dark mode."
Consider the half-states — If you support a "system" or "auto" mode, think carefully about what icon represents "use system preference."
Test with real users — Watch someone use your toggle. Do they hesitate? Do they click it and seem surprised by the result? These are signs your iconography might be backwards.

The Bigger Picture

This might seem like a small detail, but it's emblematic of a larger UX philosophy: respect your users' cognitive resources. Every interface element should reduce mental load, not add to it.

When a user glances at your theme toggle, they should instantly understand what clicking it will do. They shouldn't have to decode whether the icon represents the current state or the future state. Make it obvious. Make it intuitive. Show them where they're going, not where they are.

The page already tells them where they are.

I implemented this approach on docs.deno.com and fresh.deno.dev/docs. You can see the related pull requests and deeper implementation details in my previous post: Perfecting Dark Mode Support with Tailwind on Deno Fresh.

Let's make this the standard. Your users' brains will thank you.

The Case for a Universal Theme Toggle Standard: Show What Happens, Not What Is