Partially revert "Fix"

This reverts commit f606a5d.

Revert "Partially revert f5c2ddf and f89e10f"

This reverts commit f09a244.

lib/helpers.js

@@ -91,15 +91,14 @@ function generateBodyClass(pathname) {
 }
 
 function selectedTheme(selected) {
-    if (typeof selected === 'undefined' || selected === '') {
+    const totalThemes = files.bootswatch4.themes.length;
+    const theme = Number.parseInt(selected, 10);
+
+    if (Number.isNaN(theme) || theme < 0 || theme >= totalThemes) {
         return app.theme;
     }
 
-    const theme = Number.parseInt(selected, 10);
-
-    return theme === 0 || theme ?
-        theme :
-        app.theme;
+    return theme;
 }
 
 function getTheme(selected) {
@@ -115,25 +114,13 @@ function getTheme(selected) {
     };
 }
 
-function getThemeQuery(req) {
-    const totalThemes = files.bootswatch4.themes.length;
-    const query = Number.parseInt(req.query.theme, 10);
-
-    // Safety checks
-    if (Number.isNaN(query) || query < 0 || query >= totalThemes) {
-        return '';
-    }
-
-    return query;
-}
-
 function appendLocals(req, res) {
     const siteUrl = getCurrentSiteurl(req);
     const pageUrl = req.originalUrl;
     // OK, hack-ish way...
     const pathname = pageUrl.split('?')[0];
     const canonicalUrl = new URL(pathname, app.siteurl);
-    const theme = getThemeQuery(req);
+    const theme = selectedTheme(req.query.theme);
     const bodyClass = generateBodyClass(pathname);
 
     const locals = {