feat: user can now change the background of the whole document

[Grolaf]

This feature comes to allow user to change the background color, see the video below.

Sans.titre.mp4

• The filler is mandatory because the div do not ends at the end of the document, but at the end of the user input
• This also comes with little tricks with margins, obtained during testing. For example, if you remove the margin trick in styles.html, then you'll have a white 1px margin to the right of the document, leaving your print format unusable before cropping.

Please feel free to blame me if there's something wrong with the code.