fix: render light palettes with matching appearance

Sources/Conjure/ColorScheme.swift

@@ -10,6 +10,28 @@ struct ColorScheme {
     var glow: NSColor
     var keyBackground: NSColor
     var error: NSColor
+    enum Variant: String {
+        case light, dark
+    }
+
+    var variant: Variant? = nil
+
+    var isLight: Bool {
+        if let variant { return variant == .light }
+        return Self.relativeLuminance(background) > Self.relativeLuminance(text)
+    }
+
+    static func relativeLuminance(_ color: NSColor) -> CGFloat {
+        let rgb = color.usingColorSpace(.sRGB)!
+        return relativeLuminance(red: rgb.redComponent, green: rgb.greenComponent, blue: rgb.blueComponent)
+    }
+
+    static func relativeLuminance(red: CGFloat, green: CGFloat, blue: CGFloat) -> CGFloat {
+        func linear(_ value: CGFloat) -> CGFloat {
+            value <= 0.04045 ? value / 12.92 : pow((value + 0.055) / 1.055, 2.4)
+        }
+        return 0.2126 * linear(red) + 0.7152 * linear(green) + 0.0722 * linear(blue)
+    }
 
     static let sunset = ColorScheme(
         background: NSColor(srgbRed: 0.12, green: 0.09, blue: 0.08, alpha: 1),
@@ -49,7 +71,13 @@ struct ColorScheme {
         if theme == "tint" {
             let themeURL = directory.appendingPathComponent("theme.json")
             if let generated = try readObject(at: themeURL, missingAllowed: false) {
-                try validateKeys(generated, allowed: ["colors"], at: themeURL, prefix: "")
+                try validateKeys(generated, allowed: ["variant", "colors"], at: themeURL, prefix: "")
+                if let value = generated["variant"] {
+                    guard let name = value as? String, let variant = Variant(rawValue: name) else {
+                        throw ColorSchemeError(path: themeURL.path, reason: "variant must be \"light\" or \"dark\".")
+                    }
+                    result.variant = variant
+                }
                 try result.applyColors(from: generated, at: themeURL)
             }
         }