With your solution, I am now able to see and edit the . stb file (as in screenshot attached). But when I use this .stb to plot my file it doesn't seem to apply any plotting settings as my drawing is still yellow and green and etc instead of black/white with different line weights. But after adding this file I was able to correctly plot the drawing using the old .stb file which I still can't open. This is totally crazy.

It seems to me that the problem is that in all of my layers it says plot style 005 / 012pr / normal / etc and then on the file you sent me I see them as 5, 12, 20 etc instead of 005, 012, etc..And therefore they don't match and the .stb doesn't work. But in your screenshot it seems that you do see them as 005...what would be the proper solution...? I am not able to rename the plot styles in the .stb file. I do but after save & close they go back to normal...