Outselling all competition on River Island’s website over a record-breakingly wet Easter bank holiday weekend was a £16.99 T-shirt that reads: “It’s All Good.” Meanwhile, over at J Crew, if you missed out on last season’s “UP BEAT” slogan tee, seen on several New York fashion-week showgoers, you can now buy one that reads “ON THE BRIGHT SIDE”.

At Topshop, the top which reads “You Make Me :)” is sold out in all sizes, and the long-sleeve tee with “Be Happy” written above rainbow stripes is now only available in a size four, but you can cheer yourself up with an alternative that just reads “Good Vibes”.

T-shirts with positive slogans are the new T-shirts with random French words on them. Except it is more than that. It is not just that happiness has overtaken feminism and French as the T-shirt aesthetic of the season, it’s that the emotional tone of fashion has warmed up by several notches. Even Victoria Beckham, who more or less built a personality cult around her superhuman ability to refrain from the slightest mouth-twitch of a smile in public, modelled a T-shirt from her new collection that reads “A Dark But Happy Place” to launch her latest Victoria Victoria Beckham collection at Marks Club in Mayfair a month ago. “I wear sunglasses a lot so it’s always dark in my world, but I’m happy really,” she said.

Smiles are replacing pouts in the images selling clothes, because the optics of online retail are ever more inspired by the aesthetic of social media influencers – who tend to go for a warm, approachable vibe – rather than trying to ape cover-girl chill. (Also, when the Asos T-shirt says Smile, the sell makes more sense if the woman wearing the garment is doing exactly that.) The visuals for the streetwear collaboration between Champion and Harley Viera Newton, which will go on sale at Urban Outfitters this spring, show gaptoothed model-of-the-moment Slick Woods beaming in her cherry-print hoody.

This is a seismic shift. Being borderline morose has been the only possible way to look hip for generations. On the front row, being cool, jaded, unimpressed and generally completely over it, expressing said attitude with an icy stare, a rigid no-smiling policy and layers of black, has gradually shifted toward expressing unguarded enthusiasm via selfie-taking and wearing colour. (Brights do tend to look better on the streetstyle blogs.) On the catwalk at London fashion week, rainbow stripes were the across-the-board standout motif, from Burberry’s block-colour puffa to the sequin layer-cake dresses at JW Anderson and joyous diagonal stripes of Fyodor Golan.

The dopamine dressing that began last year with the yellow dress trend sparked by the film La La Land has stepped up a gear, with clothes that remind us in the most literal possible way that one day the rain will stop and the sun will come out. The real world doesn’t show any signs of getting less problematic, but fashion is determined to look on the bright side. This is not how it is supposed to work. Fashion is supposed to reflect the world we live in. When the economy slumps and the geopolitical picture is dark, accepted wisdom is that hemlines will dip and the colour palette will contract. We will quit peacocking around and dress either to cocoon or armour ourselves. For fashion to take it upon itself to bring the positivity at such a moment is something of a turn up for the books.

This can be seen as pure escapism, rainbows being as far beyond our grasp as those other pop cultural motifs of the moment, unicorns and mermaids. Perhaps these fashion choices represent us regressing into imaginative play – like children, or like the characters in Steven Spielberg’s new film Ready Player One, who take refuge from the dystopian reality of 2045 in a virtual world called the Oasis.

Grill My Cheese’s rainbow toastie. Photograph: Grill My Cheese

But when fashion leans towards a cheeriness that challenges rather than reflects the zeitgeist, it flags up some interesting precedents. The exception to the rule that dark times mean dark clothes is that when revolution is brewing, rebellious spirit sometimes shows up in what is worn on the streets before it bubbles over into actions.

For example: in the years immediately preceding the French revolution, the dire economic conditions in Paris were at odds with a trend for impractically high heels worn by both men and women. These became more and more vertiginous right up until Bastille Day, after which they disappeared from French fashion for several decades. Perhaps we will look back at this summer’s rainbow T-shirts as a crucial staging post in the emergence of the resistance. OK, maybe not. But it is possible, surely, that fashion’s refusal to have its vibe crushed by world events is part of a ground-level shift towards activism, or at the very least a rejection of defeatism as the only course of action. It’s not just clothes. Rainbows are happening in food, as well as fashion. This spring’s hot guilty pleasure foodie pop up, Grill My Cheese at Selfridges, is serving an eminently Instagrammable rainbow toastie with stripes of beetroot, rocket and caramelised onion melted into goats cheese.

Meanwhile, the rainbow layer cake has graduated from Instagram fantasy and is available off-the-shelf – complete with a unicorn horn fashioned from royal icing – in M&S. There is nothing more cheering than cake, and there is nothing more happy-making than a rainbow. Let them eat rainbow cake? Wait. What happened after that?