Use Mozilla's colors for itembox and tagbox background

This way, the colors play nicely with Windows themes (e.g. high contrast)

Re https://forums.zotero.org/discussion/38870/make-zotero-fully-recognize-windowsff-high-contrast-themes/ and https://twitter.com/stuffilike2know/status/504167872567189504

Much more work to be done for accessibility though.