I\'m using a custom jQuery 1.10.3 theme. I downloaded every straight from the theme roller and I have intentionally not changed anything.
I created a dialog box and I
I am having this issue as well. Here is the code that is getting inserted for the close button:

When I turn off the style="display:none:" on the button, then the close button appears. So for some reason that display:none; is getting set, and I don't see how to control that. So another way to address this might be to simply override the display:none. Don't see how to do that though.
I note that the answer posted by KyleMit does work, but makes a different looking X button.
I also note that this issue does not affect all dialogs on my pages, but just some of them. Some dialogs work as expected; other have no title (ie, the span containing the title is empty) while the close button is present.
I am thinking something is seriously wrong and it might not the time for 1.10.x.
But after further work, I discovered that in some cases the titles were not getting set properly, and after fixing that, the X close button reappeared as it should be.
I used to set the titles like this:
('#ui-dialog-title-ac-popup').text('Add Admin Comments for #' + $ac_userid);
That id does not exist in my code, but is created apparently by jquery from ac-popup and ui-dialog-title. Kind of a kludge. But as I said that no longer works, and I have to use the following instead:
$('.ui-dialog-title').text('Add Admin Comments for #' + $ac_userid);
After doing that, the missing button issue seems to be better, although I am not sure if they are definitely related.