night mode: Fix unreadable new user tips.
night mode: Fix unreadable new user tips.

Fix the border color, shadow color, arrow color, and background color
of the tips popovers for new users in dark mode.

Fix #8868.

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