app: Remove column-middle background color.
app: Remove column-middle background color.

The declaration making the background color white seems to be
unnecessary.

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