Hi. If you want to place the Open Street Map Addon as a background, this does not necessarily require CSS. Everything is in the settings - yes default way exist for this.
Switch the section to Fluid Width and add Open Street Map. All other addons you want to be over the map, place below Open Street Map , in the same section and in the same column, use the Advansed tab where you can choose width and Position Absolute for each of these addons then place them in the place in which you want via position settings.