Make the default clock colors readable with any accent

DesktopClock.Tests/ClockThemeTests.cs

@@ -3,6 +3,7 @@
 using System.Windows;
 using System.Windows.Media;
 using DesktopClock.Properties;
+using DesktopClock.Utilities;
 
 namespace DesktopClock.Tests;
 
@@ -82,4 +83,51 @@ public void Apply_SetsAppearanceFromTheme()
         Assert.Equal(1.5, settings.OutlineThickness);
         Assert.Equal(1, settings.TextOpacity);
     }
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void ThemeDefaults_AreReadableInLightAndDarkMode(bool isLightTheme)
+    {
+        // A fresh install and the System preset follow light or dark mode but not the accent, which can be any shade.
+        var (text, outer) = SystemThemeService.GetThemeDefaults(isLightTheme);
+
+        Assert.True(ClockTheme.Contrast(text, outer) >= 7, $"{text} on {outer}");
+    }
+
+    [Theory]
+    [InlineData("#3B003B", "#FFFFFF")] // Plum
+    [InlineData("#0078D4", "#FFFFFF")] // Default Windows blue keeps white text.
+    [InlineData("#D50056", "#FFFFFF")] // Dark pink
+    [InlineData("#FFB900", "#1A1A1A")] // Gold
+    [InlineData("#00CC6A", "#1A1A1A")] // Light green
+    [InlineData("#FFFFFF", "#1A1A1A")]
+    [InlineData("#000000", "#FFFFFF")]
+    public void ReadableTextOn_PicksWhiteOrDarkByContrast(string background, string expected)
+    {
+        var text = ClockTheme.ReadableTextOn((Color)ColorConverter.ConvertFromString(background));
+
+        Assert.Equal((Color)ColorConverter.ConvertFromString(expected), text);
+    }
+
+    [Theory]
+    [InlineData("#3B003B", "#FFFFFF")] // Plum keeps white text.
+    [InlineData("#FFB900", "#1A1A1A")] // Gold gets dark text.
+    public void AccentTheme_PicksReadableTextForTheAccent(string accent, string expectedText)
+    {
+        var theme = ClockTheme.CreateAccentTheme((Color)ColorConverter.ConvertFromString(accent));
+
+        Assert.Equal((Color)ColorConverter.ConvertFromString(accent), theme.OuterColor);
+        Assert.Equal((Color)ColorConverter.ConvertFromString(expectedText), theme.TextColor);
+    }
+
+    [Fact]
+    public void ReadableTextOn_IsAtLeastFourToOneOnEveryGray()
+    {
+        for (var v = 0; v <= 255; v++)
+        {
+            var background = Color.FromRgb((byte)v, (byte)v, (byte)v);
+            Assert.True(ClockTheme.Contrast(ClockTheme.ReadableTextOn(background), background) >= 4, $"gray {v}");
+        }
+    }
 }

DesktopClock/ClockTheme.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Windows.Media;
 using DesktopClock.Properties;
@@ -45,9 +46,7 @@ public static IReadOnlyList<ClockTheme> GetBuiltInThemes()
         return new[]
         {
             CreateSystemTheme(),
-            new ClockTheme(Loc.Get("ThemeAccent"), "Segoe UI", "SemiBold",
-                Color.FromRgb(0xFF, 0xFF, 0xFF), SystemThemeService.GetSystemAccentColor(),
-                backgroundEnabled: true, backgroundOpacity: 1, backgroundCornerRadius: 1, outlineThickness: 0.2),
+            CreateAccentTheme(SystemThemeService.GetSystemAccentColor()),
             new ClockTheme(Loc.Get("ThemeSmoke"), "Segoe UI", "Normal",
                 Color.FromRgb(0xF2, 0xF2, 0xF2), Color.FromRgb(0x0A, 0x0A, 0x10),
                 backgroundEnabled: true, backgroundOpacity: 0.55, backgroundCornerRadius: 1, outlineThickness: 0.2),
@@ -69,6 +68,40 @@ public static IReadOnlyList<ClockTheme> GetBuiltInThemes()
         };
     }
 
+    /// <summary>
+    /// The Accent look: the Windows accent as the background, with whichever text color reads best on it.
+    /// </summary>
+    public static ClockTheme CreateAccentTheme(Color accent) =>
+        new(Loc.Get("ThemeAccent"), "Segoe UI", "SemiBold", ReadableTextOn(accent), accent,
+            backgroundEnabled: true, backgroundOpacity: 1, backgroundCornerRadius: 1, outlineThickness: 0.2);
+
+    /// <summary>
+    /// White or near-black text, whichever stands out more on the given background, so a light accent like yellow still gets readable text.
+    /// </summary>
+    public static Color ReadableTextOn(Color background)
+    {
+        var white = Color.FromRgb(0xFF, 0xFF, 0xFF);
+        var black = Color.FromRgb(0x1A, 0x1A, 0x1A);
+        return Contrast(white, background) >= Contrast(black, background) ? white : black;
+    }
+
+    /// <summary>
+    /// The WCAG contrast ratio between two colors, from 1 for the same color to 21 for black and white.
+    /// </summary>
+    public static double Contrast(Color a, Color b)
+    {
+        static double Channel(byte value)
+        {
+            var c = value / 255.0;
+            return c <= 0.03928 ? c / 12.92 : Math.Pow((c + 0.055) / 1.055, 2.4);
+        }
+
+        static double Luminance(Color c) => (0.2126 * Channel(c.R)) + (0.7152 * Channel(c.G)) + (0.0722 * Channel(c.B));
+
+        var (lighter, darker) = (Math.Max(Luminance(a), Luminance(b)), Math.Min(Luminance(a), Luminance(b)));
+        return (lighter + 0.05) / (darker + 0.05);
+    }
+
     /// <summary>
     /// Applies this look to the given settings; the clock updates immediately.
     /// </summary>
@@ -90,7 +123,7 @@ public void Apply(Settings settings)
     }
 
     /// <summary>
-    /// The factory look: accent-colored text seeded from the Windows theme, like a fresh install.
+    /// The factory look: neutral text matching the Windows light or dark theme, like a fresh install.
     /// </summary>
     private static ClockTheme CreateSystemTheme()
     {

DesktopClock/Utilities/SystemThemeService.cs

@@ -12,7 +12,9 @@ public static class SystemThemeService
 {
     // Fallbacks match the app's existing default colors.
     private static readonly Color DefaultAccentColor = Color.FromRgb(0, 120, 215);
+    private static readonly Color LightThemeTextColor = Color.FromRgb(33, 33, 33);
     private static readonly Color LightThemeOuterColor = Color.FromRgb(247, 247, 247);
+    private static readonly Color DarkThemeTextColor = Color.FromRgb(242, 242, 242);
     private static readonly Color DarkThemeOuterColor = Color.FromRgb(32, 32, 32);
 
     // Theme and colorization values stored under HKCU.
@@ -23,7 +25,7 @@ public static class SystemThemeService
     private const string ColorizationColorValueName = "ColorizationColor";
 
     /// <summary>
-    /// Tries to build default text/outer colors from the system theme and accent color.
+    /// Tries to build default text/outer colors that match the system's light or dark theme.
     /// </summary>
     /// <remarks>
     /// Returns <c>false</c> when the system theme cannot be read (older Windows or missing keys).
@@ -36,11 +38,19 @@ public static bool TryGetThemeDefaults(out Color textColor, out Color outerColor
         if (!TryGetSystemThemeIsLight(out var isLightTheme))
             return false;
 
-        textColor = GetSystemAccentColor();
-        outerColor = isLightTheme ? LightThemeOuterColor : DarkThemeOuterColor;
+        (textColor, outerColor) = GetThemeDefaults(isLightTheme);
         return true;
     }
 
+    /// <summary>
+    /// The default text and background colors for the light or dark theme.
+    /// </summary>
+    /// <remarks>
+    /// The text doesn't use the accent color, since Windows allows any shade and a dark accent in dark mode or a light one in light mode can't be read.
+    /// </remarks>
+    public static (Color Text, Color Outer) GetThemeDefaults(bool isLightTheme) =>
+        isLightTheme ? (LightThemeTextColor, LightThemeOuterColor) : (DarkThemeTextColor, DarkThemeOuterColor);
+
     /// <summary>
     /// Whether Windows is using the light app theme; defaults to light when it can't be read.
     /// </summary>