Run Sage 100.  Enter your user name and password.  Click the Settings button.  Change the Desktop option to Classic.

That's still not "Classic".  The buttons are shaded dark grey, the font is changed, etc.

Have you tried the application scaling options?

OK.  I figured out my issue.  You have to have both the interface drop down as "Classic" and the Theme as "Classic".

Good enough!

I hated it at first too but two things won me over. (1) Ability to sort the menus and (2) Ability to increase the screen font size. 

I agree with Wayne.  Modules in alpha and sub menus in their logical order.  I love that. I also love Favorites and that you can once again collapse the menu up to the top bar only in 2018. 

The themes are tricky, because if you want to use different color themes for different companies, you are sort of giving up letting users have their own themes, like larger fonts and more contrast.