I have a similar problem but in 12.7
I've tried several themes VS Dark, Visual Studio 2017 and the cinammon theme. None of those themes apply.
I download the .dlt to the themes folder of the program and it always says to me that there are no themes and sends me to the web to download one.
I'm on W10 x64, and I've followed more or less your instructions in here.
-> Tried to change a colour and see if it sticks, and it does.
-> The edit of prefs.oxc also works fine. It stays modified.
-> Also tried the Process Explorer stuff and even in admin moden nothing turned up.
Tried double click on the theme and nothing. also right click and open with Directory opus and nothing....
I don't know what else to do.
P.S. since this is a problem with 12.7, should I make a new post?
Thanks in advance