Good morning Danilo,
Yep! - this is a major issue we all face I feel
In one sense (way) it all depends on what our clients (customers) think is fashion, and what is old, and what is new.
I feel that they may be so used to smart phones, smart TVs, and Tablets, that they expect to see quite simple and rather colourless screens - with limited (or some would say 'focused') functionality. "Fingers and King".
Certainly, the modern apps from Microsoft on my Windows 10 platform are simple and less colourful than our VO / Vulcan days. And they can be used by touch, even on my desktop PC.
It would appear that too much functionality "obvious" on the screen (WinForms?) is NOT what they expect to see. I refer to 'Windows Live Mail' and modern app 'Mail' - for a before and after comparison. The need for finger / touch input on many devices means the designers have to go to simpler layouts, with large controls- not sure why the lack of colour, may have something to do with screen lighting and battery life, but blues, white, and grey is certainly 'in' it would seem.
Sorry I have to sound so negative, but it does seem to me that we have no real room to 'wriggle'.
I personally suspect that the World or WinForms design and modern 'Touch Design' with WPF data binding, are two very different ways of looking at the Universe.
The best you can do is to try and make your WinForms screens look like what is viewed on modern devices, and leave the functionality as it is. Can you split (or share) your functionality over a few screens?
A bit / lot of research needs to be done by you first, to see what is modern and 'out there' and secondly what it is that your clients think is 'fashion'.
Whatever it is, remember, "the customer/user is always right!"
You could of course brand yourself as "Retro Danilo" and make a strength or 'selling point' out of the perceived 'weakness'
I got badly caught like this in the interface (divide) between Clipper and Windows. Functionality did NOT win the day, it had to be Windows, even though my Clipper app worked well over the network servers.
Hence my obvious and open 'stand point' in the Forum by recommending guys to write any new .NET code in 'WPF technology' and move away from WinForms. I have not hidden my views over the years on this 'chestnut' of a problem.
Remember WinForms and WPF can happily co-exist. What about redesigning one or two main screens in the WPF style with modern colours etc., etc.
Anyway, good luck, and a VERY Happy Christmas,
Phil.
Wales, UK.