Using autotools to support pre- and post-3.4 Gtk+ themes

In Gtk+ 3.4, the CSS themes were changed to require px to be used on size-based values, while before that you do not specify units.

To handle this, you can check if px is required by using:

AC_MSG_CHECKING([if GTK+ CSS themes need an explicit px unit specifier])
PKG_CHECK_EXISTS([gtk+-3.0 >= 3.4.0],
    [AC_MSG_RESULT([yes])
     AC_SUBST(GTK_PX, [px])],
    [AC_MSG_RESULT([no])
     AC_SUBST(GTK_PX, [])])

You then need to rename your styles.css to styles.css.in and add styles.css to your AC_CONFIG_FILES, e.g.:

AC_CONFIG_FILES([
    Makefile
    styles.css
])

All the areas where you need to specify px, replace with @GTK_PX@, e.g.:

GtkNotebook {
    border-width: 1@GTK_PX@ 0 1@GTK_PX@ 0;
}

Now, whenever you run ./configure, you will have a styles.css that will work on the different versions of Gtk+.