I have put a nice picture in a 800px view port as a page-background but it's not nice anymore in 320 px viewport. I thought I just had to change the properties-background of the page in 320 px, and I choose a white background but the background picture of the 800 px page is still there, but uncompletely displayed and basically even uselessly funny

Otherwise, I distributed object-texts i.e verses of a song, in the 5 central columns of 5 layoutgrids with 3 colums in a 800px view-port page. Then in 320px I did not want some columns to be visible, so I used the visible/unvisible options in the properties of the layout grids. It displays ok, but do I have to give a value to the "overflow". I decided that yes -since you mentioned it somewhere- and choose "hidden" . Is it ok to do like that ?
Thank you for all this help.