Twine, Sugarcube; CSS indv selectors not working

Twine Version: 2.3.16
Story Format: Sugarcube 2.36.1

body {
 background-color: #000000;
 color: #ffffff;
 font-size: 20px;
 font-family: "Papyrus", "Times New Roman", serif;  
 line-height: 40px;

#ui-bar {
  background-image: url("");
  opacity: 0.65;
#Dark {
  background-image: url("");

Trying to edit a specific passage (Dark passage, tagged with Intro and Dark), but changes don’t register. I’ve used Class selectors and ID selectors and still nata. Colors, opacity, etc. don’t register. I’ve checked for case sensitivity and nothing is wrong, I’ve checked the syntax and I’m pretty sure I’ve been doing it right. Just in case I’ve used different forms of syntax like passage-Dark, etc.

That’s not how you select based on passage tags.

Try something like the following:

html[data-tags~="Dark"] {
    background-image: url("");

Thanks, not a programmer. Twine has resources for this stuff, but for a writer it’s a crap shot if I understand or not.