How do I modify any fieldset defined already in extension like here

I try to reference any id in my custom file it always gives me this error
“screen HTML doesn't contain any elements that satisfies such querySelector”
How do I modify any fieldset defined already in extension like here

I try to reference any id in my custom file it always gives me this error
“screen HTML doesn't contain any elements that satisfies such querySelector”
Best answer by darylbowman
In my situation, I modified the original file to remove the ‘#’ and didn’t see any negative effects from it. Can’t recommend for production, but it may be an option.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.