Custom email background theme

Mouse over the gear in the upper right and click Themes in the drop down menu.

Thanks, but that's what I've tried multiple times. Sorry my question wasn't stated well. I'd like to know if I can add my own photo as the email background. Now, that I think about it, even the gear symbol doesn't work well, as when I click on Themes, only about 10 background theme options appear, plus a bunch of blank boxes - in which the name of the photo appears, but the box is empty! There is no photo in the box! Don't know why! This only happens in the Firefox browser, not in Chrome nor Edge!

So really 2 questions:

1) Can I add a personal photo to become email theme background?

2) What can I do in Firefox to get the empty box theme options to appear!

There doesn't appear to be a way to customize the att/yahoo webmail theme other than what is provided.

I use FF (48.0.2 on a Mac).  I see (if I counted right) about 35 themes with 9 showing at any one time.  Some positions are blank (didn't count them in my 35).  I am not sure they are truly blank or just placeholders.   If anyone was not going to see stuff I would think it would be me considering all the ad and javascript blocking I use!

Do you use any add-ons?  Maybe one of them is interfering with the page.  You could try launching FF in safe mode.  That will disable all your addons for that launch.