Add preference for higher contrast symbols

uhabits-android/src/main/res/values/strings.xml

@@ -237,6 +237,8 @@
     <string name="sync_key_already_installed">Sync key already installed</string>
     <string name="pref_unknown_title">Show question marks for missing data</string>
     <string name="pref_unknown_description">Differentiate days without data from actual lapses. To enter a lapse, toggle twice.</string>
+    <string name="pref_higher_contrast_title">Use higher contrast for symbols</string>
+    <string name="pref_higher_contrast_description">Task progress symbols will use a higher contrast font color.</string>
     <string name="you_are_now_a_developer">You are now a developer</string>
     <string name="activity_not_found">No app was found to support this action</string>
     <string name="pref_midnight_delay_title">Extend day a few hours past midnight</string>

uhabits-android/src/main/res/xml/preferences.xml

@@ -52,6 +52,13 @@
             android:title="@string/pref_unknown_title"
             app:iconSpaceReserved="false" />
 
+        <CheckBoxPreference
+            android:defaultValue="false"
+            android:key="pref_higher_contrast_enabled"
+            android:summary="@string/pref_higher_contrast_description"
+            android:title="@string/pref_higher_contrast_title"
+            app:iconSpaceReserved="false" />
+
         <CheckBoxPreference
                 android:defaultValue="false"
                 android:key="pref_checkmark_reverse_order"

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/preferences/Preferences.kt

@@ -230,6 +230,10 @@ open class Preferences(private val storage: Storage) {
         return storage.getBoolean("pref_unknown_enabled", false)
     }
 
+    fun isHigherContrastEnabled(): Boolean {
+        return storage.getBoolean("pref_higher_contrast_enabled", false)
+    }
+
     /**
      * @return An integer representing the first day of the week. Sunday
      * corresponds to 1, Monday to 2, and so on, until Saturday, which is