made flatly default theme + fixed preview image

cookbook/migrations/0017_auto_20200216_2257.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.2 on 2020-02-16 21:57
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('cookbook', '0016_auto_20200213_2335'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='userpreference',
+            name='theme',
+            field=models.CharField(choices=[('BOOTSTRAP', 'Bootstrap'), ('DARKLY', 'Darkly'), ('FLATLY', 'Flatly'), ('SUPERHERO', 'Superhero')], default='FLATLY', max_length=128),
+        ),
+    ]