theme: add dark theme with toggle

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/res/values/strings.xml

@@ -29,6 +29,9 @@
     <string name="create_activity_title">Create WireGuard Tunnel</string>
     <string name="create_empty">Create from scratch</string>
     <string name="create_from_file">Create from file or archive</string>
+    <string name="dark_theme_title">Use dark theme</string>
+    <string name="dark_theme_summary_on">Currently using dark night theme</string>
+    <string name="dark_theme_summary_off">Currently using light day theme</string>
     <string name="delete">Delete</string>
     <string name="dns_servers">DNS servers</string>
     <string name="edit">Edit</string>

app/src/main/res/xml/preferences.xml

@@ -7,4 +7,10 @@
         android:title="@string/restore_on_boot_title" />
     <com.wireguard.android.preference.ToolsInstallerPreference android:key="tools_installer" />
     <com.wireguard.android.preference.ZipExporterPreference android:key="zip_exporter" />
+    <CheckBoxPreference
+        android:defaultValue="false"
+        android:key="dark_theme"
+        android:summaryOn="@string/dark_theme_summary_on"
+        android:summaryOff="@string/dark_theme_summary_off"
+        android:title="@string/dark_theme_title" />
 </PreferenceScreen>