Support light, dark and custom color themes.

Fix the bug where the date icon was not updated when the system date has changed.
Fix the bug to allow the date icon to function properly in the private browsing mode.

Bug fix:
- Update the icon with preferred calendar day when Firefox starts.

Known issue: The day is not displayed in the icon when a private browsing/incognito profile is started. This is due to the restrictions applied in the private browsing/incognito profile.

Change log:
- Support custom fonts for Hijri and Jalali dates in the pop up view
- Change the add-on icon to display the preferred calendar day