If everyone was happy with the old theme (non-responsive Curve MC), and no one was complaining about using it on phones, then there would have been no reason to make any changes.
As RCR explained, the changes were because of Google responsiveness. Don't think anyone was complaining of the old/usual/normal theme. And I don't think people were asked if they were or weren't happy with it.
If you want a theme that's responsive for easier use on mobiles, then it'll take a bit of work to find and customize one that'll work for everyone.
It is RCR that wants/needs the responsice theme. None of us, maybe aside from you, were aware of the responsiveness issue until RCR mentioned it. Even those of us who are a bit techie don't come here thinking about those matters. We only want the board to be readable and working.
Mobile themes are something of a horror show because of all the variations in devices, compounded by the usual differences in personal preferences.
Indeed, mobile themes are a horror show. The personal preferences also aply to computer (or tablet) users. Gmail will soon be chaning again, but, as always, will allow for the use of its classic theme, that is the one I use with a personalized colour.
RCR, shinny/bright colours are a problem regardless of the colour. They tire and hurt the eyes and make things difficult. For people to remain on a site/spend time on a site, the site must be comfortable to the eyes (and head, some tones and shades and light intensity cause some people terrible headaches).
The more mate soft grey-green (the original multi-colour) is comfortable and does not tire the eyes.
Also to remember that, like Ursa said, some colours are not good to be seen at night because they affect sleep. It was the case of the blue themes.
Can you create a mirror board, with all the users and features, but have it on a different url, or on a beta/experimental page? That way you wouldn't have to be testing on the real board, rather on the replica.
I have mobile-desktop running here, currently out-of-box.
Whatever that thing is, it looks pretty much like all the other blue designs. It hurt the eyes, is too bright, weird small font, etc. If you are going to do new tries, please find a away of leaving the option to revert to curve multi-colour if we so wish.
Sometimes good things fall apart so better things can fall together. (Marilyn Monroe)