I understand they're trying to move people off of Classic for some likely practical reason or another, but it's a weird turn off the way they're trying to do it.
Being like "Hey, try out Retro" and it's just... Also Modern.

Like... if I wanted to use modern: I would use modern.

Also a little annoying that they don't provide any sort of preview, clicking it just automatically sets your setting to Retro, and then you have to go find where that is configured to undo it.

Takes 1 click to change to Retro, takes several clicks and your password to change back.
Very Dark Pattern. Very Amazon. Very Modern "we're optimizing for efficiency so you can interact with systems more and people less" type energy.
Very "tag blocking is here so people don't have to look at the wrong types of queers" anymore type energy.
Not really vibing with it.

If they'd just tell us why the change needs to happen, I'd maybe understand. But the sheer opaqueness, and refusal to explain the problem, is just, well.

Why should I ever move away from the Classic theme then? You (the FA staff, not Tehweenus) never explained to me what the problem with the theme is. The only actual downside with it is lacking the tag filter - which sucks, but oh well, I'll have that over a substantially worse feeling and looking GUI overall.

The way they are handling this is just really really shitty.

It leaves one to assume that they cannot be transparent about their aims because doing so would provoke a schism.
But I'd really prefer to believe it is simply a matter of incompetence and a small team of volunteers doing what they believe is best to maintain the relevance of one of Furry culture's most important and historical hubs.

If there wasn't a schism beforehand, they've engineered one now anyway.

I'll switch to modern when it doesn't suck.

Honestly, I think Modern is better. BUT to fix your issue, I recommend using Ublock Origin and blocking the rainbow so it's not there anymore

By this point, it's staying up to see how long they want to annoy users that dislike the Modern theme styling. A barometer of how much they do or do not respect not annoying portions of their userbase needlessly.

I wouldn't have known this banner existed if I hadn't check. Been using UBlock for years.

I'm having the same gripe with Modern as it stands, this new setup really doesn't add anything that sparks an interest in trying Modern for me. Classic still just feels good enough for what I want.