Add support for using a platform-specific look and feel

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In principle, it should be possible to very easily add support for using a 
platforms-specific look and feel in the Java Swing application. A quick guide 
to this Java feature can be found in Oracle's Java Tutorials:

http://docs.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html

Ponomar currently uses the default "Metal" theme in Windows and Linux. If I 
understand correctly, Apple's implementation of the JVM for OsX will 
automatically use an appropriate OsX theme as the default.

The Oracle JVM provides support for the following platform-specific themes for 
Windows, Linux, and Unix:

com.sun.java.swing.plaf.gtk.GTKLookAndFeel
com.sun.java.swing.plaf.motif.MotifLookAndFeel
com.sun.java.swing.plaf.windows.WindowsLookAndFeel

The Motif theme is very antique looking, and can be ignored.

The GTK theme looks good in Linux desktop environments such as Gnome and XFCE, 
which use the GTK+ widget library. GTK+ applications look a little alien in 
other desktops like KDE.

I tested the Windows and GTK themes by using the applicable command line 
options. The themes basically look good, but there were a few problems that 
would require some troubleshooting if we want to support this feature.

1. The background colors for normal, feast, and fast days only work with the 
default theme. Thus all days in the calendar look the same, except for the red 
foreground color for Sundays, and the yellow background color for the current 
date. This defect would have to be fixed if we wanted to support 
platform-specific themes.

2. The system's menu font settings are ignored. For example, the menu font used 
with the Windows look and feel is Times New Roman, whereas ordinarily it would 
be a sans serif font such as Segoe UI. This is just a quirk, but we might want 
to fix it if it were easy to do.

There is no vital need to support platform-specific themes now. I was just 
tinkering with it to see if it worked at all.

Original issue reported on code.google.com by ps008v...@gmail.com on 6 Feb 2015 at 5:28

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[deleted comment]

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I should test to see whether the OsX theme has the same problem as the Windows 
and GTK+ themes (namely, that the day background colors for normal, feast, and 
fast days don't work).

Original comment by ps008v...@gmail.com on 6 Feb 2015 at 6:40