added oled theme and theme presets

BalanceFlutter/test/oled_preview_test.dart

@@ -0,0 +1,44 @@
+import 'dart:io';
+import 'dart:ui' as ui;
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'widget_test.dart' as fixtures;
+
+void main() {
+  setUpAll(fixtures.loadReadableFont);
+  setUp(fixtures.mockStorage);
+  for (final size in [const Size(390, 844), const Size(1280, 900)]) {
+    testWidgets('OLED renders black pixels at $size', (tester) async {
+      final store = (await tester.runAsync(() => fixtures.createStore()))!;
+      await tester.runAsync(() => store.setThemePreset('oled'));
+      final key = GlobalKey();
+      await fixtures.boot(tester, store, size, boundaryKey: key);
+      for (final page in [0, 4]) {
+        await fixtures.navigate(tester, page);
+        await tester.runAsync(() async {
+          final boundary =
+              key.currentContext!.findRenderObject()! as RenderRepaintBoundary;
+          final image = await boundary.toImage(pixelRatio: 1);
+          final rgba = await image.toByteData(
+            format: ui.ImageByteFormat.rawRgba,
+          );
+          // Left edge of the scaffold/rail below the app bar, away from text.
+          final pixel = (110 * image.width) * 4;
+          expect(rgba!.getUint8(pixel), 0);
+          expect(rgba.getUint8(pixel + 1), 0);
+          expect(rgba.getUint8(pixel + 2), 0);
+          expect(rgba.getUint8(pixel + 3), 255);
+          final png = await image.toByteData(format: ui.ImageByteFormat.png);
+          final dir = Directory('/tmp/balance-oled-preview')
+            ..createSync(recursive: true);
+          File(
+            '${dir.path}/${size.width.toInt()}-${page == 0 ? 'overview' : 'settings'}.png',
+          ).writeAsBytesSync(png!.buffer.asUint8List());
+          image.dispose();
+        });
+        expect(tester.takeException(), isNull);
+      }
+    });
+  }
+}