portico: Fix trapsarent gradient styling inconsistencies.
portico: Fix trapsarent gradient styling inconsistencies.

Instead of using the `trapsarent` keyword, which is interpreted
as Safari as black with an opacity of 0%, re-use the gradient colors
themselves in order to lead to a single color gradient. This allows
for the homepage to look the same regardless of browser.

Fix #11985.

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