File gtkcssprovider.c contains a detailed description of the style sheets that
can be parsed. One missing detail is that it's nowhere mentioned that the first
character of an identifier is in some situations used to determine what kind of
entity is identified. Specifically it's assumed in the source code that the
name of a widget class begins with an uppercase letter.

gtkcssprovider.c, function parse_selector
   line 1764:  else if (g_ascii_isupper (scanner->value.v_identifier[0]))
                  Then it's a widget class.
   line 1805:  else if (g_ascii_islower (scanner->value.v_identifier[0]))
                  Then it's a region.

gtkcssprovider.c, function parse_rule
   line 3316:  else if (prop[0] == '-' &&
                        g_ascii_isupper (prop[1]))
                  Then it's a widget class + property name registered with
                  gtk_widget_class_install_style_property(),
                  else if it's not a registered property, then it's ignored
                  without a warning message or anything similar.

All class names that are registered by C++ classes in gtkmm get the name prefix
gtkmm__ which is incompatible with the parsing in gtkcssprovider.c. It's not
possible to use style sheets to set values of properties of custom widgets,
defined by such a class.

If the restriction on class names and other names will remain, it should be
documented. The name prefix can be changed in gtkmm 3, but from the description
in gtk+ it should be possible to conclude that changing is necessary.