editing style.css on a macbook

hello,

i'm using darkmode on a macbook, because of that my elements are very hard to see. i read a few threads about editing the style.css file to fix this. however the only style.css file that i can find is in "documents/Qelectrotech samples/styles/style.css". editing this file does nothing, even deleting it does nothing. i also made sure that the option "use system colors" is disabled. i'm starting to think there should be a styles.css file somehwere else but i cannot seem to find it. anyone has an idea?

thanx in advance!