dark-mode: Change `background-image` and `background-color` to background.
dark-mode: Change `background-image` and `background-color` to background.

There isn’t any reason for either of these mention code paths to be
using specified background types, so convert them all to background so
inheritance is more simple.

dhcrzf/zulip,shubhamdhama/zulip,eeshangarg/zulip,punchagan/zulip,brainwane/zulip,dhcrzf/zulip,kou/zulip,hackerkid/zulip,dhcrzf/zulip,timabbott/zulip,tommyip/zulip,hackerkid/zulip,zulip/zulip,mahim97/zulip,showell/zulip,kou/zulip,showell/zulip,hackerkid/zulip,showell/zulip,zulip/zulip,andersk/zulip,rishig/zulip,mahim97/zulip,dhcrzf/zulip,brainwane/zulip,tommyip/zulip,timabbott/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,showell/zulip,zulip/zulip,jackrzhang/zulip,rishig/zulip,jackrzhang/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,timabbott/zulip,tommyip/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,rishig/zulip,synicalsyntax/zulip,rishig/zulip,kou/zulip,synicalsyntax/zulip,timabbott/zulip,synicalsyntax/zulip,showell/zulip,kou/zulip,andersk/zulip,shubhamdhama/zulip,dhcrzf/zulip,synicalsyntax/zulip,timabbott/zulip,tommyip/zulip,kou/zulip,andersk/zulip,dhcrzf/zulip,punchagan/zulip,brainwane/zulip,mahim97/zulip,shubhamdhama/zulip,zulip/zulip,jackrzhang/zulip,eeshangarg/zulip,tommyip/zulip,andersk/zulip,brainwane/zulip,shubhamdhama/zulip,timabbott/zulip,brainwane/zulip,mahim97/zulip,mahim97/zulip,hackerkid/zulip,rishig/zulip,kou/zulip,punchagan/zulip,rht/zulip,dhcrzf/zulip,kou/zulip,punchagan/zulip,andersk/zulip,jackrzhang/zulip,synicalsyntax/zulip,tommyip/zulip,jackrzhang/zulip,tommyip/zulip,showell/zulip,andersk/zulip,jackrzhang/zulip,synicalsyntax/zulip,punchagan/zulip,mahim97/zulip,rht/zulip,brainwane/zulip,rishig/zulip,zulip/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,rht/zulip,shubhamdhama/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,shubhamdhama/zulip,eeshangarg/zulip,brainwane/zulip,synicalsyntax/zulip,jackrzhang/zulip,rishig/zulip,hackerkid/zulip