Add custom theme class

lib/themes/custom_theme.dart

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+import 'package:flutter/material.dart';
+
+class CustomTheme {
+  final Color primaryColor;
+  final Color primaryText;
+  final Color kunyomiColor;
+  final Color kunyomiText;
+  final Color onyomiColor;
+  final Color onyomiText;
+  final Color meaningColor;
+  final Color meaningText;
+  final Color errorColor;
+  final Color backgroundColor;
+
+  const CustomTheme({
+    this.primaryColor,
+    this.primaryText,
+    this.kunyomiColor,
+    this.kunyomiText,
+    this.onyomiColor,
+    this.onyomiText,
+    this.meaningColor,
+    this.meaningText,
+    this.errorColor,
+    this.backgroundColor,
+  });
+}

lib/themes/dark.dart

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+import 'package:flutter/material.dart';
+
+final darkTheme = ThemeData(
+  primaryColor: Color(0x56D926),
+);