User story

1. As an end-user, I want to use a modern flat design calendar component
2. As an app developer, I want to upgrade the existing calendar to get bug fixes without breaking the current layout.

Spec.

1. Calendar supports 4 themes: iceblue (default), breeze , dark, wcag
2. apply a theme according to org.zkoss.calendar.theme.preferred an app developer specified in his zk.xml. If not specified, it shows the default theme. For the users who apply a custom theme, the built-in resolving rule always resolves a custom theme to the default theme which is not expected. So this property allows app developers can enforce a specific theme.
3. Calendar contains 4 dsp theme files
4. If the above property doesn't exist, Calendar determine its theme according to the property org.zkoss.theme.preferred in zk.xml
5. Because zk supports more themes including classic themes and theme pack themes. Calendar contains a built-in theme resolving rule:
   1. If the specified theme matches one of the 4 built-in themes, the calendar just applies the matched theme. e.g. breeze
   2. if one of the dark themes of the theme pack is specified, the calendar resolves to dark, including ruby, amber, emerald, aquamarine, montana, violet and spaceblack, cardinal, mysteriousgreen, zen and their corresponding compact themes
   3. If one of the WCAG themes (or WCAG compact) is specified, the calendar will apply wcag theme
   4. If either silvertail or sapphire is specified, the calendar will apply breeze theme
   5. If the preferred theme name doesn't match any of the names listed above, the calendar will apply iceblue