diff --git a/src/c3nav/settings.py b/src/c3nav/settings.py
index 2bc8b7e0..eac0ce07 100644
--- a/src/c3nav/settings.py
+++ b/src/c3nav/settings.py
@@ -575,7 +575,7 @@ BASE_THEME = {
     'css': {
         'initial': config.get('theme', 'css_initial', fallback='#ffffff'),
         'primary': config.get('theme', 'css_primary', fallback='#9b4dca'),
-        'logo': config.get('theme', 'css_logo', fallback='#9b4dca'),
+        'logo': config.get('theme', 'css_logo', fallback=None),
         'secondary': config.get('theme', 'css_secondary', fallback='#525862'),
         'tertiary': config.get('theme', 'css_tertiary', fallback='#f0f0f0'),
         'quaternary': config.get('theme', 'css_quaternary', fallback='#767676'),
diff --git a/src/c3nav/site/themes.py b/src/c3nav/site/themes.py
index c282c85b..2f4948c0 100644
--- a/src/c3nav/site/themes.py
+++ b/src/c3nav/site/themes.py
@@ -31,6 +31,12 @@ def modify_vars(css_vars):
         css_vars[key] = fn(rgb)
     for key in remove:
         del css_vars[key]
+    falsy_vars = []
+    for key, val in css_vars.items():
+        if not val:
+            falsy_vars.append(key)
+    for key in falsy_vars:
+        del css_vars[key]
 
 
 def make_themes(theme_models):