In my themes section, I don't have the option to SELECT THE OLD CUSTOMIZED THEME.
Yes, you do. You just don't know it.
Unless you went in there and actually DELETED the theme via FTP, then it's still there. One of the themes listed in your Themes section is the old theme.
You cannot lose a theme by accident. It requires FTPing in and deleting the thing manually. So your theme is STILL THERE. Really. You know how you switched themes the first time? Do that again, but switch to one of the other themes. One of them is the old customized theme.
See, a theme has a name. When they customized it, they probably didn't change the picture or the name. So one of those themes in the Themes section, one that you think is not the custom theme, is actually the custom theme. All you have to do is click on them, then see if that's the one it is by looking at the site.