Here are some small patches that fix some deprecated functions when building the GTK3 version. I have not (yet?) tackled the color-scheme.c file, nor the widget-theme.c file because:
1. In GTK 3.4+, the color chooser is changed and I personally don't like it so I wouldn't mind if we stick to the old deprecated one; I'll leave this up to you.
2. I can't find a proper alternative to gtk_rc_get_theme_dir, so I have left it untouched.