Thats sounds like a glitch. My inventory wont let me delete themes, accidentally or intentionally.
Also dont know why that girl has stopped selling themes. That sounds like another glitch.
Just so you know I dont think themes actually do anything except waste money.
Better to spend it on all the gadgets, those are a heck of a lot more useful to you. The First Aid station is especially helpful.
Wish I could help but this sounds like an issue for a patch to fix. You could also try checking gamefaqs.com and seeing about cheat codes. Maybe you could add the item to your inventory. But I dont think simply having it will change your house. I think the game runs a script when you make the purchase.
Try looking into the scripts. I know with Morrowind and Oblivion you could use the control console to manually run scripts and make old actions happen again. Perhaps you can rerun the script where she offers you household themes.
You may wish to find a hardcore Fallout 3 mod site and investigate this.
http://falloutmods.wikia.com/wiki/Fallout_3_Mods
http://fallout3mods.net/
http://www.fallout3nexus.com/