Customize Login pop-up box?

Development
4

Kind of a tricky question. So, I want to change the login pop-up box text.
I am currently using CSS to replace the “Log in to your account” text with “Login or Register here!”

.login-subheader {
  visibility: hidden;
}
.login-subheader:after {
  content:'Login or Register here!'; 
  visibility: visible;
}

The problem is, the result is this, where it should be to the left like the original text is…

wwww
wwww810×459 24.1 KB

But that’s because the original, “Log in to your account” is simply hidden, not gone, so what’s really there is:

qqq
qqq788×423 20.7 KB

So is there a way to just directly change the HTML here instead of using whacky CSS tricks?