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I have installed several GTK styles, none shows up.
Btw, I googled and it seems that starting in plasma 5.20, GTK styles are not working anymore (even in other distros, like Kubuntu)
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What if you try something like "lxappearance" to change GTK theme?
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Worksforme™ how exactly did you "install several GTK styles" and where did you put them. Check whether you have any errors starting the GTK kcm in a terminal.
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