Ok let me start out by saying that Steve has most probably gone off to sleep now and I see the problem with the link he gave and have to say that I am really not sure where the link was supposed to point to but I will try and find out.
However – let’s see if we can talk you through this.
At some point in the past you created a custom SP theme and called it ‘my theme’. I do not know why you did this but it is possible it was because of a WP theme conflict that needed to be dealt with and by making a custom theme instead of editing one of the supplied themes it meant changes would be preserved on future updates.
As Steve said right at the start, WP – in it’s 3.6 release – annoyingly made changes to the dialog (the popup we use) that required extra styling be put in place in our supplied themes. This meant that any one using a custom theme would not get those changes automatically and would need to make those changes manually. The link to the codex article he sent you detailed exactly what those changes are (along with other, optional changes… note a document like this is available after every new SP release).
Now – you mist have made your custom theme for a reason. So by abandoning that and using spDefault (out of the box) you are also abandoning any customised code or special styling that was made to your own ‘my theme’ and obviously deemed necessary at some point in the past.
So – we know that applying the popup dialog changes described in the SP 5.3.0 changes document will solve the initial reported issue with popups. We do not know if it solves the problem with your title – which might have been the original reason for creating the custom theme in the first place. Only you know why you made it.