Issue with setting the margins for the text

[Shanny]

I'm trying to set the margins in the Viewer. I know I can do that in Preferences > Page Layout but the thing is I want the Viewer to display 2 pages at the same time and if I add margins in Page Layout the margin between the two pages is doubled because it applies margins for both the left and the right page. I don't want that. I want the margin between the pages to be same as the margin at the other sides of pages.
I managed to accomplish that using the inspector but the change I make there is not saved and applied when I reopen the book.
I also tried using custom CSS in Viewer > Preferences > Styles but I can't get it to work. I copied the selector from the Inspector (#view > div > div > div > div:nth-child(2)) but it doesn't apply any of the CSS code I write. I can't just apply CSS to generic div element because I need to style a specific div element that contains the text. How do I do that?