ModularContent/MetaBox.php and 'modern' admin color scheme

[BCITWebServices]

Problem:
ModularContent/MetaBox::print_admin_theme_css() throws "PHP Notice: Undefined offset: 3" whenever the user has the 'modern' admin style chosen, which only has 3 colors available in it.

Proposed fix:
'modern' should be added to the list of other color schemes that are using index 2 here.

[defunctl]

@BCITWebServices we'd certainly accept a PR for this update, if you're willing!

Thanks