I see there's a lot going on here: Add CheckedColor and UncheckedColor properties to CheckBox (#3635) by jaideepj2004 · Pull Request #3752 · mit-cml/appinventor-sources · GitHub
Can we expect an update on this soon? Personally, I don't think a background color needs to be adjustable (neither for the Checkbox.Checked true nor false). But it definitely shouldn't stay that ugly green.