How adress CSS classes with spaces in their names?

I have a problem applying CSS code for some IDs and Classes. Looking for help.

I’m using the Breeze Dark NC theme, with some custom CSS adjustments, which works fine.
However, the Breeze theme doesn’t get it right everywhere. Especially prominent are the potentially lesser used/observed pages, such as two-factor authentication login page, as well as “forgotten password” page form and a few other error messages. On such pages, the background is shown as light-colored text on light-colored background. Looks weird.

I’m trying to remedy this, by making the form background color dark, using the Custom CSS NC app.
I don’t succeed with this, and I believe my failure is because I fail to address the CSS class/id of the target I wish to change. The CSS class/id identifier has spaces in their names, like this: <div class="body-login-container update two-factor">

Addressing this code, in Custom CSS app, by writing .body-login-container update two-factor { background: #030303; } doesn’t change the background color of the class. Neither does it work if I exchange the spaces with dots/periods - .body-login-container.update.two-factor { background: #030303; }

I am not sure how to address the desired CSS class/id, besides of course for tedious time-consuming systematic trial-and-error ‘throw everything at the wall and see what sticks’.

If anyone may have an idea what I’m failing to see/understand, it would be appreciated to hear. Perhaps this has something to do with CSS hierarchy or inheritance?