Replace deprecated css variable

The `assignee` styles are long deprecated and will be removed anytime.
Use the equivalent replacement added in change Ifba52ecf25a1d8c3705d60655d9bcb45fa2669bd
and available since Gerrit v3.8.

This also fixes how the banner appears in dark mode on those Gerrit
versions since the dark mode variable override was removed.

Change-Id: Iaed9a658904d6566b8ec67ddc47705e479642c66
1 file changed