Add new gtk3 theme based on Greybird 3.22.9

[ddelabru]

Bluebird is not compatible with the latest versions of GTK+ 3, so I replaced the gtk3 portion of the theme with the Greybird gtk3 theme and modified it to bring back some of Bluebird's distinctive features: the blue menubar that connects seamlessly with the window manager themes, the same "selected" background color used by the Bluebird gtk2 theme, and an XFCE panel with more blue in it.

[ddelabru]

@knomepasi, I mentioned this in #shimmer. I'm currently using it as my XFWM + Gtk theme

[ddelabru]

@ochosi, let me know how this looks. I'm using it as my XFWM + GTK theme right now, and may try to link some screenshots here in the discussion

[ddelabru]

Here's one screenshot for starters:

[ochosi]

@ddelabru Hey, that looks neat!
I don't think I really have the time and energy to maintain this theme right now, but if you do please go ahead and take over. I've discussed this also with @knomepasi and he's fine with that too.

Let us know what you think.

[ddelabru]

@ochosi, @knomepasi, I'm up to try maintaining the theme going forward. I'll be hanging out in the # shimmer channel on freenode periodically if anyone needs to discuss it more.

[ochosi]

@ddelabru Invited you as a contributor, so you should be able to push and do releases now.

Welcome aboard!

[knomepasi]

Welcome from me too!

[jsane-h8ms]

Thank you guys so much for this! I've been pushing off upgrading my distro for so long. Been thinking about paying someone to port Bluebird...

[srd424]

+1 to the thanks for this work! Haven't had a chance to install yet but screenshots look great. Also reassuring to know that I'm not the only one who puts off upgrading due to potential cosmetic change :)

[hroncok]

Thanks for this!

A note: The nonactive window menus text looks a bit weird.