theme: fix submenu separator contrast

Fixes #2203
6 jobs for !1003 with wip/jimmac/fix-submenu-separators in 9 minutes and 30 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #592452
check_commit_log

00:00:38

passed #592454
eslint

00:00:56

passed #592453
js_check

00:00:29

 
  Build
passed #592455
build

00:07:20

 
  Test
passed #592456
test

00:01:14

passed #592457
test-pot

00:00:58