fix: page flickers when user's dark/light pref differs to theme's default

Fixes typo calling canChangeTheme().

Fix eb8748ff17
Fix #1601

exampleSite/config/_default/params.toml

@@ -16,7 +16,7 @@
 theme = "minimal"
 
 # Enable users to switch between day and night mode?
-day_night = false
+day_night = true
 
 # Override the theme's font set (optional).
 #   Latest font sets (may require updating): https://sourcethemes.com/academic/themes/