Recently a question came up, where a user changed the Default theme on Project Online (Site Settings >> Change the look), to something else. Then when they wanted to bring back the default theme, it was no where to be found.
This piqued my curiosity and I started poking around. True enough, there was no theme named the “Default Theme”, so that one can return back to it.
Since there were only so many themes, I just decided to try each one of them and figure out which one actually matched the default theme.
To cut a long story short, the easiest way for you to get back to the default theme is to select one of the following themes ( Orange, Green, Grey, Office) and select the Color Palette, that looks like this:
Make sure the Fonts are set to Seattle and Site Layout is set to Seattle.
Just to complete the loop, you could use ANY theme, as long as remove the image, and set the Colors to the one I show above, and then make sure the Fonts are set to Seattle and Site Layout is set to Seattle. They will all revert to the default theme that Project Online starts with.
So, I think Microsoft’s reason could have been that, since it is all the essentially the same theme, a separate theme called Default Theme is probably unnecessary. Still, I think that it would have been a good addition, so that users would not need to figure it out.