diff --git a/source/Ox.UI/js/Core/Theme.js b/source/Ox.UI/js/Core/Theme.js
index 4adebd01..3a6ce785 100644
--- a/source/Ox.UI/js/Core/Theme.js
+++ b/source/Ox.UI/js/Core/Theme.js
@@ -20,7 +20,7 @@ Ox.Theme = (function() {
         classNames && Ox.forEach(classNames.split(' '), function(className) {
             if (Ox.startsWith(className, 'OxTheme')) {
                 theme = className.replace('OxTheme', '');
-                theme = theme[0].toLowerCase() + theme.substr(1);
+                theme = theme[0].toLowerCase() + theme.slice(1);
                 return false; // break
             }
         });