For #5519: Add telemetry for theme settings.

app/metrics.yaml

@@ -431,3 +431,20 @@ pro_tips:
           The tip code of tip being clicked.
           Can be one of fresh_look_tip, allow_list_tip, request_desktop_tip.
         type: string
+
+preferences:
+  user_theme:
+    type: string
+    description: >
+      A string that indicates the theme.
+      Can be one of LIGHT, DARK, or FOLLOW DEVICE.
+      Default is FOLLOW DEVICE.
+    bugs:
+      - https://github.com/mozilla-mobile/focus-android/issues/5519
+    data_reviews:
+      - https://github.com/mozilla-mobile/focus-android/pull/5526
+    data_sensitivity:
+      - interaction
+    notification_emails:
+      - [email protected]
+    expires: "2022-10-01"

app/src/main/java/org/mozilla/focus/telemetry/GleanMetricsService.kt

@@ -25,6 +25,7 @@ import org.mozilla.focus.GleanMetrics.GleanBuildInfo
 import org.mozilla.focus.GleanMetrics.LegacyIds
 import org.mozilla.focus.GleanMetrics.MozillaProducts
 import org.mozilla.focus.GleanMetrics.Pings
+import org.mozilla.focus.GleanMetrics.Preferences
 import org.mozilla.focus.GleanMetrics.Shortcuts
 import org.mozilla.focus.GleanMetrics.TrackingProtection
 import org.mozilla.focus.ext.components
@@ -113,6 +114,25 @@ class GleanMetricsService(context: Context) : MetricsService {
         TrackingProtection.hasAnalyticsBlocked.set(settings.hasAnalyticsBlocked())
         TrackingProtection.hasContentBlocked.set(settings.hasContentBlocked())
         TrackingProtection.hasSocialBlocked.set(settings.hasSocialBlocked())
+
+        // theme telemetry
+        val currentTheme =
+            when {
+                settings.lightThemeSelected -> {
+                    "Light"
+                }
+                settings.darkThemeSelected -> {
+                    "Dark"
+                }
+
+                settings.useDefaultThemeSelected -> {
+                    "Follow device"
+                }
+                else -> ""
+            }
+        if (currentTheme.isNotEmpty()) {
+            Preferences.userTheme.set(currentTheme)
+        }
     }
 
     private fun getDefaultSearchEngineIdentifierForTelemetry(context: Context): String {