Crash in gtk_style_context_resolve_color()

Epiphany is crashing on startup after yesterday's changes, presumably from !7430 (merged). Here is the top of the backtrace:

Gtk:ERROR:../gtk/gtkcsscolorvalue.c:2191:gtk_css_color_value_get_rgba: assertion failed: (color->type == COLOR_TYPE_COLOR)
Bail out! Gtk:ERROR:../gtk/gtkcsscolorvalue.c:2191:gtk_css_color_value_get_rgba: assertion failed: (color->type == COLOR_TYPE_COLOR)

Thread 1 "epiphany" received signal SIGABRT, Aborted.
__pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0)
    at pthread_kill.c:44
44	      return INTERNAL_SYSCALL_ERROR_P (ret) ? INTERNAL_SYSCALL_ERRNO (ret) : 0;
(gdb) bt
#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=6, no_tid=no_tid@entry=0)
    at pthread_kill.c:44
#1  0x00007ffff6b42a23 in __pthread_kill_internal (threadid=<optimized out>, signo=6)
    at pthread_kill.c:78
#2  0x00007ffff6aeaaae in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#3  0x00007ffff6ad2882 in __GI_abort () at abort.c:79
#4  0x00007ffff7cb507c in g_assertion_message
    (domain=domain@entry=0x7ffff73aa1f2 "Gtk", file=file@entry=0x7ffff73be7bc "../gtk/gtkcsscolorvalue.c", line=line@entry=2191, func=func@entry=0x7ffff742fe70 <__func__.2> "gtk_css_color_value_get_rgba", message=message@entry=0x5555561430b0 "assertion failed: (color->type == COLOR_TYPE_COLOR)")
    at ../glib/gtestutils.c:3389
#5  0x00007ffff7d21b67 in g_assertion_message_expr
    (domain=domain@entry=0x7ffff73aa1f2 "Gtk", file=file@entry=0x7ffff73be7bc "../gtk/gtkcsscolorvalue.c", line=line@entry=2191, func=func@entry=0x7ffff742fe70 <__func__.2> "gtk_css_color_value_get_rgba", expr=expr@entry=0x7ffff73e6dc0 "color->type == COLOR_TYPE_COLOR") at ../glib/gtestutils.c:3415
#6  0x00007ffff7126fbf in gtk_css_color_value_get_rgba (color=color@entry=0x555555903bf0)
    at ../gtk/gtkcsscolorvalue.c:2191
#7  0x00007ffff7204abe in gtk_style_context_resolve_color
    (context=<optimized out>, color=0x555555903bf0, result=0x7fffffffc4d0)
    at ../gtk/deprecated/gtkstylecontext.c:797
#8  gtk_style_context_lookup_color
    (context=<optimized out>, color_name=color_name@entry=0x7ffff7f8021c "theme_base_color", color=color@entry=0x7fffffffc4d0) at ../gtk/deprecated/gtkstylecontext.c:830
#9  0x00007ffff7f6d432 in ephy_web_view_constructed (object=0x5555561179c0 [EphyWebView])
    at ../embed/ephy-web-view.c:3662

Here is what Epiphany does:

  context = gtk_widget_get_style_context (GTK_WIDGET (web_view));
  if (gtk_style_context_lookup_color (context, "theme_base_color", &color))

It crashes in the call to gtk_style_context_lookup_color(). @alicem says the color is not being resolved properly.