First off forgive me, just seen the dev responses not others posts, 12 pages is too much for me to read!
My first thoughts on the colour of the UI is that it felt... hmm. But then the comparison side by side made me think that perfection is a difficult thing to obtain, and I don't think the new one is perfection but it is a darn sight nicer to look at than the current one. I get the logic for the original theme, it looks "Rome'y", and it probably would have been ok if we had variants for different cultures (broadly, One for india, one for middle east, one for celtic, maybe a couple more).
Even though trying primary and secondary colours for nations UI is probably ugly for the majority of nations (no guarantee of a good match), it'll be nice if the red, black and the teal like colour for buttons were modifiable, I think we could work with that and make a set for major (if not all) nations through mods, and would give a slightly more unique feel to the game as different nations. The text colour for all three to be editable would be nice too (gold on red, white on black, white on teal seems to be current plan - but full editing of these would help)
I'm preferring the senate look, and the family section - feels more like what I expect it to be.
I'm wondering if the text backgrounds in the first two images needs to be more opaque by a few percent.
I'm also wondering if the province front tab still needs some work on that left hand side, the right side looks a lot better with it sub-tabbed though.
I don't know how much people pay attention to culture/religion on the province tab, perhaps if there was unique icons for culture groups and religion, you could hover over the icons for specifics and save some space - lot of icon work though (but nothing that isn't done on other PDX games).