The upright pink triangle on this conservative dress in The Rocky Horror Picture Show (1975) is so emblematic of the film's premise. The Nazis marked queer men/trans women with the pink triangle pointing down. The pink triangle pointing up is a queer reclaimed design to convey community support.
The pink triangle was used by early gay liberation movements as a deliberately uncomfortable symbol. Other gay people like Harvey Milk disliked its use.
It became more widespread after 1987 (12 years after the film) when the Silence = Death project used it in their iconic AIDS awareness poster.
Andy Campbell wrote in his "Queer X Design" history of queer graphics,
"In design terms, their innovation was to appropriate the pink triangle, a symbol of Nazi concentration camps, and to flip it so it stood on its base—conveying the strength of an otherwise stigmatized social position."
It's a great example of the kind of inversions used throughout the film. The dress is conservative, but visibly marked as transexual. The pink triangle is a symbol of hate that's been flipped around into a symbol of queer community and struggle.
The whole commentary structure of the film was to take the most extreme conservative backlash fears about how queer liberation will destroy society and then make the cheesiest, campiest, drag horror B movie out of it as a way of throwing it all back in their faces.
It's one of those details that's harder to notice the subtle significance of now that when when the film was made because the symbol's use has come and gone, and it's hard to imagine what the world was like when it was pushing boundaries. Our time is a product of it.