Changing the Word template

Some of these Microsoft apps are enough to drive you mad! Several years ago when I got my last computer, I went through the nightmare of trying to change the normal template for Word, and just now I've been going through it again. I've finally got the template fixed the way I want it, which includes a black background color, and the template works. But now I find that all of my old Word files (several hundred) suddenly have a white background, which they never had before. It hurts my eyes, and I can't use it. I'm gonna go crazy unless I find a way either to change them all back to the way they were, or else a way to make them all conform to the new template. Can either of those things be done, without having to do it one file at a time, which would take a month?

This is crazy! Apparently it has something to do with the "Never change page color" option. If I set that option on, my new normal template works but all my existing documents turn white. If I turn it off, the existing documents go back to their old colors, but I can't choose what colors I want for the new normal template! All I want is to choose my new normal template (for future documents) and leave my old documents unchanged. Can't that be done? Or, if the old documents changed to the new template, that would be fine. But they're turning white instead.

Not sure what version of Office you have but...,
in the latest version you can switch viewing modes for existing docs...

I have Office 16. I've tried both dark mode and light mode. I've been trying every possible combination of options and settings, and nothing works. I can get the new template the way I want it, but the minute I change it all the colors disappear from my existing documents and they all turn white.

Not a solution but it may help with the older documents in knowing which version of Word created them. Up through Office 2013 the individual apps within the suite had different version numbers, Office 2016 was the first to have a similar version number of 16. Most times a .doc file can be saved as 97-2003, the change to the new format occurred with Office 2007 which is identified by the x added to the file extension as in .docx. Office 2010 was the first the available as either 32-bit/x86 or 64-bit/x64. Documents created in older versions of a program may not always work the features in newer versions. If older documents can be opened perhaps they can be resaved with the newer features but as always recommended do the testing on a second copy so the first is not lost.

It looks like the mode is the problem. My old documents were created in "Dark Mode" and if I use any other mode now, their colors disappear. But if I try to create the new template in Dark Mode, its background color comes out wrong. If I ask for black (#000000) I get white, and if I ask for white (#FFFFFF) I get a sort of shining silvery gray. No matter what numbers I enter, I can't get a plain black background in their so-called "Dark" mode!

This is a fine mess! Apparently by "Dark Mode" they mean a mode to use in a dark room, and it has bright colors on the screen. And by "Light Mode" they mean a mode to use in a brightly-lit room, and it has dark colors on the screen. So light really means dark, and dark really means light. And we're supposed to figure that out for ourselves!

Anyway, I'm screwed no matter what I do. If I select any mode except Dark, all my existing files appear with white backgrounds, which my eyes can't take. And if I select Dark mode, I get light colors in all new files or templates that I create.

Isn't there ANY way out of this mess?