Hello,
You’re almost there. Looking at what you’re posting, I think you just made a typo there where you made the background color transparent. You’ve put a “#” in front of the word transparent.
Keep in mind that you also don’t need to copy all of that, but just write the portion that changes what you want. So this, should do the trick for you:
.standard-slider .media-full .content {
background-color: transparent;
}
Also, make sure this is going in the custom.css file of your Child theme and not the style.css file.