catch error if attempting to select or deselect a non-existent theme.

Author: expikr

src/theme/book.js

@@ -361,13 +361,17 @@ function playground_text(playground, hidden = true) {
             });
         }
 
-        var previousTheme = get_theme();
+        var previousTheme = get_theme().replace(/\W+/g, '_').toLowerCase();
+        var selectedTheme =       theme.replace(/\W+/g, '_').toLowerCase();
         if (store) {
             try { localStorage.setItem('mdbook-theme', theme); } catch (e) { }
         }
 
-        html.classList.remove(previousTheme.replace(/\W+/g, '_').toLowerCase());
-        html.classList.add(theme.replace(/\W+/g, '_').toLowerCase());
+        try { 
+            html.classList.remove( previousTheme );
+            html.classList.add(    selectedTheme ); 
+        } catch (e) { }
+        
         updateThemeSelected();
     }
 

src/theme/index.hbs

@@ -84,8 +84,10 @@
             if (theme === null || theme === undefined) { theme = default_theme; }
             var html = document.querySelector('html');
             html.classList.remove('no-js')
-            html.classList.remove('{{ default_theme }}')
-            html.classList.add(theme.replace(/\W+/g, '_').toLowerCase());
+            try { 
+                html.classList.remove('{{ default_theme }}');
+                html.classList.add(theme.replace(/\W+/g, '_').toLowerCase()); 
+            } catch(e) { }
             html.classList.add('js');
         </script>