fix: change the template selection to use light and dark checkmark

packages/flutter_availability/lib/src/config/availability_options.dart

@@ -144,6 +144,8 @@ class AvailabilityColors {
     this.textDarkColor,
     this.textLightColor,
     this.templateWeekOverviewBackgroundColor,
+    this.templateColorLightCheckmarkColor,
+    this.templateColorDarkCheckmarkColor,
     this.templateColors = const [
       Color(0xFF9bb8f2),
       Color(0xFF4b77d0),
@@ -179,6 +181,14 @@ class AvailabilityColors {
   /// If not provided the text color will be the theme's text color
   final Color? textDarkColor;
 
+  /// The light color variant of the checkmark icon when selecting a color for
+  /// a template
+  final Color? templateColorLightCheckmarkColor;
+
+  /// The dark color variant of the checkmark icon when selecting a color for
+  /// a template
+  final Color? templateColorDarkCheckmarkColor;
+
   /// The color of the background in the template week overview that creates a
   /// layered effect by interchanging a color and a transparent color
   /// If not provided the color will be the theme's [ColorScheme.surface]

packages/flutter_availability/lib/src/ui/widgets/color_selection.dart

@@ -1,3 +1,5 @@
+import "dart:math";
+
 import "package:flutter/material.dart";
 import "package:flutter_availability/src/util/scope.dart";
 
@@ -38,35 +40,96 @@ class TemplateColorSelection extends StatelessWidget {
           spacing: 8,
           runSpacing: 8,
           children: [
-            for (var color in colors.templateColors)
-              GestureDetector(
-                onTap: () => _onColorClick(color),
+            for (var color in colors.templateColors) ...[
+              _TemplateColorItem(
+                color: color,
+                selectedColor: selectedColor,
+                onColorSelected: onColorSelected,
+              ),
+            ],
+          ],
+        ),
+      ],
+    );
+  }
+}
+
+class _TemplateColorItem extends StatelessWidget {
+  const _TemplateColorItem({
+    required this.color,
+    required this.selectedColor,
+    required this.onColorSelected,
+  });
+
+  /// Callback for when a color is selected or deselected
+  final void Function(int?) onColorSelected;
+
+  final Color color;
+
+  final int? selectedColor;
+
+  @override
+  Widget build(BuildContext context) {
+    var availabilityScope = AvailabilityScope.of(context);
+    var options = availabilityScope.options;
+    var colors = options.colors;
+
+    /// If the color is selected, deselect it, otherwise select it
+    void onColorClick(Color color) => onColorSelected(
+          color.value == selectedColor ? null : color.value,
+        );
+
+    var checkMarkColor = _hasHighContrast(color)
+        ? colors.templateColorLightCheckmarkColor
+        : colors.templateColorDarkCheckmarkColor;
+
+    var icon = selectedColor == color.value
+        ? Icon(Icons.check, color: checkMarkColor)
+        : null;
+
+    return GestureDetector(
+      onTap: () => onColorClick(color),
       child: Container(
         width: 40,
         height: 40,
         decoration: BoxDecoration(
           color: color,
           borderRadius: options.borderRadius,
-                    border: Border.all(
-                      color: color.value == selectedColor
-                          ? Colors.black
-                          : Colors.transparent,
-                      width: 1,
         ),
+        child: icon,
       ),
-                  child: selectedColor == color.value
-                      ? const Icon(Icons.check)
-                      : null,
-                ),
-              ),
-          ],
-        ),
-      ],
     );
   }
+}
 
-  /// If the color is selected, deselect it, otherwise select it
-  void _onColorClick(Color color) => onColorSelected(
-        color.value == selectedColor ? null : color.value,
-      );
+/// Computes the relative luminance of a color
+/// This is following the formula from the WCAG guidelines
+double _relativeLuminance(Color color) {
+  double channelLuminance(int channel) {
+    var c = channel / 255.0;
+    return c <= 0.03928 ? c / 12.92 : pow((c + 0.055) / 1.055, 2.4).toDouble();
+  }
+
+  return 0.2126 * channelLuminance(color.red) +
+      0.7152 * channelLuminance(color.green) +
+      0.0722 * channelLuminance(color.blue);
+}
+
+/// Computes the contrast ratio between two colors
+/// This is following the formula from the WCAG guidelines
+double _contrastRatio(Color color1, Color color2) {
+  var luminance1 = _relativeLuminance(color1);
+  var luminance2 = _relativeLuminance(color2);
+  if (luminance1 > luminance2) {
+    return (luminance1 + 0.05) / (luminance2 + 0.05);
+  } else {
+    return (luminance2 + 0.05) / (luminance1 + 0.05);
+  }
+}
+
+/// Returns true if the color has high contrast with white
+/// This is following the WCAG guidelines
+bool _hasHighContrast(Color color) {
+  const white = Color(0xFFFFFFFF);
+  return _contrastRatio(color, white) > 4.5;
 }