feat(desktop): platform/appearance/accent overrides for local preview

Adds three override hooks so a single-OS developer can preview every
platform's theming without VMs:

  AMETHYST_PLATFORM=GNOME ./gradlew :desktopApp:run
  AMETHYST_APPEARANCE=light ./gradlew :desktopApp:run
  AMETHYST_ACCENT=#3584E4 ./gradlew :desktopApp:run

Each override accepts either an env var or a `-Damethyst.<key>=<value>`
JVM property (forwarded from the gradle invocation via build.gradle.kts).

- PlatformInfo: `amethyst.platform` accepts MACOS, WINDOWS, GNOME, KDE,
  LINUX_OTHER, UNKNOWN. Adds a `host` accessor for code that needs the
  real underlying OS (for a future fallback hook).
- PlatformAppearance: `amethyst.appearance` accepts light/dark.
- PlatformAccent: `amethyst.accent` accepts `#RRGGBB`, `RRGGBB`, or any
  libadwaita accent name (blue, teal, green, yellow, orange, red, pink,
  purple, slate).

Window chrome stays native to the host OS — overriding the platform
swaps in-app theming only, not the AWT title bar — so on a Mac you
still see traffic lights even when previewing the GNOME theme.

https://claude.ai/code/session_01NufduPfZvYQVYwLkbCjCUo

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/platform/PlatformAccent.kt

@@ -38,14 +38,41 @@ import com.vitorpamplona.quartz.utils.Log
  * identity and looks intentional rather than broken.
  */
 object PlatformAccent {
-    fun systemAccent(): Color =
-        when (PlatformInfo.current) {
+    /**
+     * Resolves the OS accent color, with an override hook for testing:
+     * `-Damethyst.accent=#3584E4` (system property) or `AMETHYST_ACCENT=blue`
+     * (env var). Accepted: `#RRGGBB`, `RRGGBB`, or any libadwaita accent name
+     * (blue, teal, green, yellow, orange, red, pink, purple, slate).
+     */
+    fun systemAccent(): Color {
+        forcedAccent()?.let { return it }
+        return when (PlatformInfo.current) {
             Platform.MACOS -> macOSAccent()
             Platform.GNOME -> gnomeAccent()
             Platform.KDE -> kdeAccent()
             Platform.WINDOWS -> windowsAccent()
             else -> DefaultPrimary
         } ?: DefaultPrimary
+    }
+
+    private fun forcedAccent(): Color? {
+        val raw =
+            System.getProperty("amethyst.accent")
+                ?: System.getenv("AMETHYST_ACCENT")
+                ?: return null
+        val trimmed = raw.trim()
+        // Hex (#RRGGBB or RRGGBB) — long form for clarity
+        val hex = trimmed.removePrefix("#")
+        if (hex.length == 6 && hex.all { it.isDigit() || it in 'a'..'f' || it in 'A'..'F' }) {
+            val v = hex.toLongOrNull(16) ?: return null
+            return Color(
+                red = ((v shr 16) and 0xFFL).toInt(),
+                green = ((v shr 8) and 0xFFL).toInt(),
+                blue = (v and 0xFFL).toInt(),
+            )
+        }
+        return gnomeAccents[trimmed.lowercase()]
+    }
 
     // macOS named accent colors (NSColor controlAccentColor variants).
     // Index matches AppleAccentColor defaults value; -1 = Multicolor (use highlight).

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/platform/PlatformAppearance.kt

@@ -41,14 +41,38 @@ import java.awt.event.WindowFocusListener
  * system theme see Amethyst follow within a second of bringing the window forward.
  */
 object PlatformAppearance {
-    fun isSystemDark(): Boolean =
-        when (PlatformInfo.current) {
+    /**
+     * Resolves the OS dark/light preference, with an override hook for testing
+     * on a single machine: `-Damethyst.appearance=light|dark` (system property)
+     * or `AMETHYST_APPEARANCE=light|dark` (env var).
+     *
+     * When the platform is overridden but appearance isn't, this calls the
+     * override platform's detection routine. On a Mac forced to GNOME the
+     * `gsettings` shell-out won't exist and it falls through to the GNOME
+     * default (dark) — pass `AMETHYST_APPEARANCE=light` to flip it.
+     */
+    fun isSystemDark(): Boolean {
+        forcedAppearance()?.let { return it }
+        return when (PlatformInfo.current) {
             Platform.MACOS -> isMacOSDark()
             Platform.GNOME -> isGnomeDark()
             Platform.KDE -> isKdeDark()
             Platform.WINDOWS -> isWindowsDark()
             else -> true
         }
+    }
+
+    private fun forcedAppearance(): Boolean? {
+        val raw =
+            System.getProperty("amethyst.appearance")
+                ?: System.getenv("AMETHYST_APPEARANCE")
+                ?: return null
+        return when (raw.lowercase()) {
+            "dark" -> true
+            "light" -> false
+            else -> null
+        }
+    }
 
     private fun isMacOSDark(): Boolean {
         val out = exec("defaults", "read", "-g", "AppleInterfaceStyle") ?: return false

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/platform/PlatformInfo.kt

@@ -39,7 +39,20 @@ enum class Platform {
 }
 
 object PlatformInfo {
-    val current: Platform by lazy { detect() }
+    /**
+     * The detected (or overridden) UI platform — drives all per-OS theming.
+     *
+     * Override for testing on a single machine via `-Damethyst.platform=GNOME`
+     * (system property) or `AMETHYST_PLATFORM=GNOME` (env var). Accepted values
+     * are the [Platform] enum names, case-insensitive: MACOS, WINDOWS, GNOME,
+     * KDE, LINUX_OTHER, UNKNOWN.
+     */
+    val current: Platform by lazy { override() ?: detect() }
+
+    /** The actual host OS, ignoring any override. Useful for shell-out helpers
+     * that should always hit the real underlying system (e.g. accent detection
+     * falling back to the Mac's value when the override platform's CLI is absent). */
+    val host: Platform by lazy { detect() }
 
     val isMacOS: Boolean get() = current == Platform.MACOS
     val isWindows: Boolean get() = current == Platform.WINDOWS
@@ -47,6 +60,14 @@ object PlatformInfo {
     val isKde: Boolean get() = current == Platform.KDE
     val isLinux: Boolean get() = current.isLinux
 
+    private fun override(): Platform? {
+        val raw =
+            System.getProperty("amethyst.platform")
+                ?: System.getenv("AMETHYST_PLATFORM")
+                ?: return null
+        return runCatching { Platform.valueOf(raw.uppercase()) }.getOrNull()
+    }
+
     private fun detect(): Platform {
         val osName = System.getProperty("os.name", "").lowercase()
         return when {