The fills are referenced by the windows and doors (and all library objects) by their index numbers, NOT their names. If you rename something Background Fill, that will do nothing... the name is only for your convenience.
You must look at Attribute Manager (the 2 column version) to see what the index numbers are. You can restore some of the essential fills by creating a fresh file from the template (the out-of-the-box OOTB experience) and saving it as a PLN. Load that PLN in the right side of Attribute Manager to look through to see where you have created havoc by renaming or re-indexing required attributes.
As the original poster noted... and you can find many tutorials in (very) old posts here on the Community... never change the base attribute index numbers, names, contents... if you want to add new things... add them at the very end of each list. Many people pick index 500 as a random place to start adding their own things. Also, if you change pens 1 through 9, 91, and some others I can't remember, then library parts will not look correct when placed form the library with their default pen settings.
Only expert users should adjust existing attributes. Become an expert user by exploring them in a test file, never in a real project 👍
EDIT: PS to all - sorry if my post looks redundant. I just found that the OP has posted the same question in December, and Nathan had answered. I've merged that other thread into this one. Please do not start new threads on the same original question 🙂
One of the forum moderators • AC 26 USA and earlier • macOS Ventura 13.2.1, iMac Pro 10-Core, Radeon Pro Vega 64