This is the 4422nd most frequent Polish word.
iście
"Iście" means "truly" or "really" in English, used to emphasize something.
Here, 'iście' acts as an adverb enhancing the adjective 'artystyczne' (artistic), conveying a sense of 'truly' or 'genuinely' in relation to the quality described.
To jest iście artystyczne dzieło.
This is a truly artistic work.
In this sentence, 'iście' adverbially modifies 'królewskim' (royal), emphasizing the nature of the celebration in an almost regal manner.
Świętowali jego sukces w iście królewskim stylu.
They celebrated his success in a truly royal style.
Here, 'iście' functions as an adverb to magnify the attribute 'w stylu retro lat 70' as genuinely or authentically in the retro style of that era.