theme: Make the overview search box background slightly transparent

The search box looks a bit lost without any non-transparent elements
around it, make it slightly transparent to match the fully transparent
panel better.

GNOME/gnome-shell!404
4 jobs for !404 with rescue-transparent-panel in 8 minutes and 13 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Review
passed #244795
check_commit_log

00:03:24

 
  Build
passed #244796
build

00:02:40

 
  Test
passed #244797
test

00:02:07

passed #244798
test-pot

00:02:07