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Halogen

Halogen

Runtime theme generation for Compose Multiplatform

Halogen turns natural language into complete Material 3 themes at runtime. Give it a prompt like "warm coffee shop" or "neon cyberpunk" and it generates colors, typography, and shapes, all cached so the LLM only runs once per prompt.

On Android, it can run entirely on-device with Gemini Nano. On iOS, Desktop, and Web, plug in any cloud LLM.

https://github.com/user-attachments/assets/d744b0db-8179-4bc0-89c8-1c7318533c78

Imagine

  • A weather app where the entire UI shifts to match the forecast - sunny yellows, stormy grays, sunset gradients
  • A community platform where every subreddit, channel, or group gets its own generated look and feel
  • A music player that themes itself to the album art or genre - jazz gets warm tones, electronic gets neon
  • An e-commerce app where each brand or product category has a distinct visual identity, generated on the fly
  • A reading app that adapts its palette to the mood of the content - thriller, romance, sci-fi

Halogen makes all of this possible with a single resolve() call.

Quick Start

// 1. Build the engine
val halogen = Halogen.Builder()
    .provider(GeminiNanoProvider())
    .cache(HalogenCache.memory())
    .build()

// 2. Generate a theme
halogen.resolve(key = "coffee", hint = "warm coffee shop vibes")

// 3. Apply it
HalogenTheme { App() }

That's three lines to go from a text prompt to a full Material 3 theme. See the Quick Start guide for setup details.

Installation

Most apps need three modules:

implementation("me.mmckenna.halogen:halogen-core:0.1.0")
implementation("me.mmckenna.halogen:halogen-compose:0.1.0")
implementation("me.mmckenna.halogen:halogen-engine:0.1.0")

Then add a provider and optionally a persistent cache:

// Gemini Nano on-device provider (Android only, min SDK 26)
implementation("me.mmckenna.halogen:halogen-provider-nano:0.1.0")

// Room KMP persistent cache (Android, iOS, JVM - not wasmJs)
implementation("me.mmckenna.halogen:halogen-cache-room:0.1.0")

How It Works

The LLM generates 6 seed colors + typography/shape hints. Halogen expands those into 49 M3 color roles, 15 text styles, and 5 shape sizes using pure-Kotlin HCT color science. One call produces both light and dark schemes - toggling dark mode is instant, no second LLM call.

resolve("coffee", "warm coffee shop")
    → cache HIT?  → return instantly
    → cache MISS? → LLM generates seeds → expand to full M3 theme → cache → apply

Results are cached by key, so the LLM is never called twice for the same prompt.

Configuration Presets

Control how seed colors get expanded into palettes:

Preset Style
Default Balanced M3 tonal spot - good for most apps
Vibrant Bolder, more saturated
Muted Desaturated, calm
Monochrome Single-hue variations
Punchy High-energy, high-contrast
Pastel Soft and airy
Editorial Strong primary, neutral everything else
Expressive Colorful - even the neutrals are tinted 
Halogen.Builder()
    .provider(myProvider)
    .config(HalogenConfig.Vibrant)
    .build()

All presets are in HalogenConfig.presets if you want to build a UI picker.

Platform Support

Platform LLM Provider Persistent Cache
Android Gemini Nano (on-device) or cloud Room
iOS Cloud providers Room
Desktop (JVM) Cloud providers Room
Web (WasmJs) Cloud providers -

All platforms get in-memory LRU caching and full Compose Material 3 support.

Learn More