Give me access to your site so I can troubleshoot the problem you're having.
I can't duplicate it here or on my development server at all.
The moment I changed my default style to Ui.X2, you changed to it, my style stayed the same, and your postbit changed.
And back to Dark as the default...
The problem is your users actively selected the Default XenForo style in XF 1.x. This will throw off the initial counts in XF 2.x if the Default XenForo style is not user selectable. Doing what I said to do above, resets the old XF 1.x default style selections to the current default style for your site.