theme: update toggle switch design

- for the short term, just replace the SVG assets

See #841 for background
4 jobs for !496 with wip/jimmac/image-based-switches-322 in 4 minutes and 2 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Review
passed #274123
check_commit_log

00:00:35

 
  Build
passed #274124
build

00:02:17

 
  Test
passed #274125
test

00:01:09

passed #274126
test-pot

00:01:09