How can I add a CSS class for
page-level using Sitecore SXA
.
You cannot add classes where you want to via Creative Exchange, because the positions you marked are grid specific.
You can add classes via Creative Exchange only where you see class
tag with add-your-css-classes-here
token inside
You will find more about it here: Change a site design using Creative Exchange
Those that you highlighted are defined here: /sitecore/system/Settings/Feature/Experience Accelerator/Bootstrap/Grid Definition
in Placeholders Styles
section. Technically you can change them, but it is not recommended
(SXA installation will overwrite it anyway during upgrade). They are applied to every site using this grid.
How about adding them into body
section if you want it to be page specific?
They will be stored in Body Css Class
field for every page
That should let you achieve your goal.