classic: Fixup panel styling after gnome-shell changes

We don't want a transparent panel in the classic session (this is
already taken care of by the existing panel background-color overrides),
and we want to avoid the new pill-shaped hover/focus indicators in the
classic session.

For the gnome-shell changes, see
gnome-shell!1397.

Part-of: <!153>
5 jobs for !153 with fixup-after-panel-changes in 44 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Pre Review
passed #1086285
check_commit_log

00:00:18

passed #1086286
check-merge-request

00:00:11

 
  Review
passed #1086288
eslint

00:00:16

passed #1086287
js_check

00:00:12

 
  Build
passed #1086289
build-bundles

00:00:26