Fix the `treeitem-expanded`/`treeitem-collapsed` images in dark-mode (PR 12666 follow-up)
Fix the `treeitem-expanded`/`treeitem-collapsed` images in dark-mode (PR 12666 follow-up)

I completely missed updating these in PR 12666; sorry about that!

timvandermeij/pdf.js,Snuffleupagus/pdf.js,showpad/mozilla-pdf.js,caremerge/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js,macroplant/pdf.js,xavier114fch/pdf.js,Shoobx/pdf.js,macroplant/pdf.js,xavier114fch/pdf.js,nawawi/pdf.js,Shoobx/pdf.js,showpad/mozilla-pdf.js,Shoobx/pdf.js,timvandermeij/pdf.js,nawawi/pdf.js,Snuffleupagus/pdf.js,mozilla/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,mozilla/pdf.js,caremerge/pdf.js