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),
+        ),
+    ]

cookbook/models.py

@@ -12,7 +12,7 @@ class UserPreference(models.Model):
     THEMES = ((BOOTSTRAP, 'Bootstrap'), (DARKLY, 'Darkly'), (FLATLY, 'Flatly'), (SUPERHERO, 'Superhero'))
 
     user = models.OneToOneField(User, on_delete=models.CASCADE, primary_key=True)
-    theme = models.CharField(choices=THEMES, max_length=128, default=BOOTSTRAP)
+    theme = models.CharField(choices=THEMES, max_length=128, default=FLATLY)
 
 
 class Storage(models.Model):

cookbook/templatetags/theming_tags.py

@@ -20,7 +20,7 @@ def theme_url(request):
         else:
             raise AttributeError
     except AttributeError:
-        return static('themes/bootstrap.min.css')
+        return static('themes/flatly.min.css')
 
 
 @register.simple_tag