Themes are used to configure the way how the window manager looks. Things like fonts, colors, border sizes, button pixmaps can be configured. Together these things form a theme.
Theme files are searched in the
libpath/themes
subdirectories.
These directories contain other directories that contain related theme files and their .xpm files. Each theme file specifies fonts, colors, border sizes, ...
The theme to use is specified using Theme configuration option or
-t command line options in the preferences file.
Name of the theme to use. Both the directory and theme file name must be specified.
If the theme directory contains a file named fonts.dir created my mkfontdir the theme directory is inserted into the X servers font search path.
http://icewm.themes.org/ contains various nice themes created by users of IceWM.