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
Merge pull request #531 from aurimasv/high-contrast

Use Mozilla's colors for itembox and tagbox background