theme-switch: Improve theme switch logic (#86)

- removes unnecessary local-storage item removal
- less inline script on build
- assumes `defaultTheme as auto when value is not `dark` or `light`
- should work according to table at docs

layouts/partials/footer.html

@@ -64,11 +64,5 @@
         }
     })
 
-</script>
-{{- else -}}
-{{/*  case where owner disables theme button after deployment, this resets the stored theme  */}}
-<script>
-    localStorage.removeItem("pref-theme");
-
 </script>
 {{- end }}