Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
var slash = mw.config.get('wgPageName').indexOf('/');
var norm = mw.config.get('wgPageName').substr(0, slash) + mw.config.get('wgPageName').substr(slash).toLowerCase();
var test = 'User:' + mw.config.get('wgUserName') + '/myskin.';
var ext;
if (norm == test + 'js') ext = 'js';
else if (norm == test + 'css') ext = 'css';
if (ext != null) window.location.href = window.location.href.replace(/\/myskin.(css|js)/i, '/' + skin + '.' + ext);