It took me a few sittings to get through this video. It was worth it, thanks Ross.
Why have GUI themes gone backwards? (& high emotions against non-conforming UIs)
In short:
(1) GUI frameworks (win32, qt*, gtk*, etc) have become much more difficult to theme for. Constantly changing targets, horridly complicated codebases, etc. When you make a theme it's a pain to start and it doesn't stably last. This is an important reason why many of the theme sites (for both Win & Linux) have not had much added to them since the late 2000's.
(2) Audiences now automatically react negatively to any sort of UI experimentation. People are getting constantly burned by bad UI changes that they can't opt out of (eg Google & Microsoft products) so now they hate all UI changes regardless of who they are from. Any sort of difference or change is "OMG THIS IS HORRIBLE I CAN'T COPE" as if we now have an uncanny valley of UI appearance. This is getting really psychologically entrenched, I can't emphasise enough how much of an issue this is.
I recently came across someone who creates crazy UIs as part of the experience of her games & programs:
Unfortunately they were getting pushback from people, which I think is a bit sad to do to those who experiment, suggesting that they somehow hurt society by daring to publish their fun little programs. My response to the page I just linked goes into some more detail:
QuoteUIs are an interesting area.
Every time I’ve attempted to use something like GTK or QT I’ve balked at the complexity. I’ve always wondered why it is so and considered whether or not I should try writing my own.
The “internet opinion consensus” is that I shouldn’t. In this era of twitter I’m beginning to think the internet opinion consensus is more of a self-fueled illusion than I thought.
Nathalie: do what you want and enjoy yourself. No-one is forced to use it and no group is made worse off by you releasing it (unless you’re a big company like Google, at which point people are forced to tolerate anyway :P).
Some approximate psychological insight gathering into why people might have knee-jerk reactions against custom UIs:
(1) A lot of 2000’s era software (like Antivirus) used custom UIs and people hated them. I think a combination of “this program needs to be utilitarian”, “this UI is getting in my way” and “this UI looks like the product of a marketing department, not an artist” were at play here. This is still strong in many people’s memories.
(2) Users are conditioned to only see a few UI styles. Anything else tends to end up sitting in an uncanny valley of bad or old. This often comes up in Linux-related forums where users complain that some applications look different to others and how it is “jarring”. I used to be in this boat too, coming from Windows many years back with its better UI consistency, but I discovered over time that the perception of the problem is worse than the actual problem. Humans are more adaptive than what they think, they’re just not used to exercising it with computer UIs.
(3) Turbulent toolkits and their histories. People don’t like to think about UIs, toolkits or systems because they bring bad memories. Opinions of GTK3 devs and the great divide made since GTK2, complexity and corner-cases of win32, the intrusiveness of build systems like those that Qt require, java complexity in general, etc. People have forgotten that UIs can be fun because their memories are so tied up in the big projects and their problems.
Unfortunately this all leads to people feeling strong emotions and acting before they think. Over something like UIs that, on the outside, seem like a mundane topic.
Expect more high emotions from UI discussions
If there is something people hate more than UI problems, its when their UIs get changed from underneath them. This makes people feel vile right to their core, especially if they feel any sort of ownership or comfort with their computer.
Ross: even the slightest discussion of UIs will bring up an extreme fear in people that you are somehow affiliated with those that do this 
Don't let this misunderstanding end your world.
Commandline vs GUI
I use both extensively. One is not universally better than the other, but you are on the right path by saying that one points out faults in the other.
Comparisons and competition are good things, if there was only one (eg only CMDline or only GUI) then we would be much much worse off. Just as we need more GUI variants and experimentation to help show people how bad the stuff we already have is.
Case in point: Windows powershell. The fact it can do things that you can't do in the UI anymore (eg remove Windows 10 pre-installed crap) shows the UI had failed, not that powershell is any good.