I upgraded to macOS Mojave and switched to dark mode.
I use Accessibility Keyboard. I am wondering if the keys on it are supposed to look like this in Light mode.
Honestly, I don't know if they're supposed to, but they do, & have done since the first beta. Still the same in the latest beta 10.14.2
Sorry, some confusion... the picture in the OP isn't the Accessibility keyboard, it's the "Show Keyboard Viewer" keyboard, which used to be white & is now as above.
From High Sierra
The Accessibility keyboard seems to have been the same since High Sierra [can't test Sierra] & didn't exist in El Capitan.