I would suggest setting "ZK_THEME_USE_FONT_ICON_FOR_IMAGE" = 'Y' as default. The CSS icon theme looks nicer and does not have this multicolour/multi-style look of the default theme's icons.
It gives a more consistent feel-and-look to the user.
For customized themes, they would only need to set this SysConfig back to 'N'
Attached the patch to this ticket for your consideration.