The highlight groups that control the default background color are "
Normal" and "
NonText". Unfortunately these are "global" highlights and cannot be "localized" on a per-window basis.
However, here is a truly ugly kludgy example:
highlight ActiveWindow guibg=lightblue
autocmd WinEnter * call matchadd('ActiveWindow', '.*', 10, 1682)
autocmd WinLeave * call matchdelete(1682)
doautocmd BgHighlight WinEnter -
Since this is just an example, you'll have to modify this to suit your individual needs, such as adding a ctermbg=... highlight along with the guibg=...
The drawback is that this will only highlight the background of existing text in the files, not the whole line/window. (Like I said, ugly and kludgy.)