Not using Nuendo / Cubase myself but I am a passionate sound designer; I can't help wonder if the decision by Steinberg is more driven by Ableton than that what drove Microsoft.
Because you see; sometimes having a dull flat gray screen actually works out quite well. Ableton Live (Suite 8) (link) is an DAW which existed for 10 years now and as you can notice by the logo and screenshots on that page: its interface is completely grey. And before anyone wonders about the "colorful blocks"; that isn't the program itself, those merely represent your audio data.
By default Ableton Live only uses grey for its interface combined with yellow to indicate selected options (system options, instrument options as well as channel on/off), red to indicate a record mode and blue to indicate an individual selected track and the main "individual" volume. And finally orange to indicate a "special" option; if a track has been set to "monitoring" (it continues to pick up audio input, no need to arm it) then the indicator will be orange. Orange is also used in the interface of some instruments (for example to be able and define a waveform in their Operator synthesizer) as well as volume and balance controls (dials).
But that's it! All audio channels have the same dull grey color, the only one which is shown to be light grey is the selected track. All instruments and effects sit in a grey box which apart from a yellow selection bar is completely grey; even the icons representing folders and different type of objects (presets, instrument/effect object, 'grouped' object).
The main issue here should be obvious: what works for some doesn't have to work for others.
Within the context of Live I actually enjoy such an interface because it helps me focus on what matters to me. If I'm recording audio then I don't care about good looks or such; all I care for is that as much CPU power as possible is used to process my recording as good as possible and in the mean time having some good indicators to see what is going on.
That is what Ableton gives me; the program itself is flat grey and my data is colored so I can always immediately see whats going on.
Needless to say but as much as I enjoy this design in Ableton (IMO its a key feature) I hate it in Visual Studio. In VS it just doesn't add up for me; different programs, different uses.