Credit to Guy Stevens for some base elements of the hangouts theming (http://freestyler.ws/style/129200/hangouts-black-v2-0).

Best combined with:
FT DeepDark add-on by steva (Stefano Roselli) (Firefox Only)

Dark Fusion - Google by DeathTBO - http://freestyler.ws/style/118354/dark-fusion-google

Google Hangouts Web Messenger (Firefox only)

DeepDark Theme by actualmanx (Chrome only) https://chrome.google.com/webstore/detail/deep-dark/eeffcpnmcmfdfnaadpnkldhkcjjiihcf

DeepDark Scroll Bars extension by actualmanx (Chrome only) https://chrome.google.com/webstore/detail/deep-dark-scrollbars/feeaojjpfjaggengmoigjlckepbmkane

2/10/2017 - New group hover and new group name text input color fixed.
1/26/2017 - Removed google contacts support. Now Hangouts only.
8/26/2016 - Fix last update and also remove initial background before hover for hangouts contact list first entry.
8/22/2016 - Unread message header contact name text now lighter.
8/21/2016 - Themed background for sending picture after image is selected and number of participants superscript at top of message while not in focus.
8/11/2016 - Darkened text for "You are in this call" message text during video call.
8/8/2016 - Themed dialogs that pop up when no connection is available. Also colored background of status when another person is typing in a hangouts conversation (thanks Devo).
7/23/2016 - Themed some missing call list items in Hangouts.
6/9/2016 - Themed new contact popup thanks to Devo.
6/3/2016 - Fixed background of status in hangouts message box which states "user isn't on Hangouts right now. he'll/she'll see your messages later
4/5/2016 - Fixed background color while loading page, fixed hangouts sms icon (thanks Devo).
12/19/2015 - Added a couple of missed icon inversions.
12/8/2015 - Unread message text color now set to white.
12/6/2015 - Left side panel selected group indicator now uses new contact button color.
12/3/2015 - Added option for new contact button color and new contact button hover color.
11/25/2015 - Shifted left panel down a bit and main panel down to line up with each other. Inverted missed icon for adding frequent contact. Changed default accent colors.
11/18/2015 - Lightened the placeholder text inside input boxes. Tweaked input box background color for new contact edit info popup to match camouflaged style of default colors.
11/14/2015 - Inverted a couple more missed icons in hangouts message header. Themed message settings and group hangouts member listing/add person sections. Lightened some contact search text elements. Darkened stickers background during selection. Themed the hangouts page slideout menu, invites section, and settings section.
11/13/2015 - Inverted a couple more missed icons in group chat hangouts message, also inverted disabled notifications icon. Modified background color of new contact name in new message recipients list.
11/8/2015 - Minor updates. Inverted some missed icons. Inverted some missed new contact info edit popup. Darkened some hangouts video call list/search items.
11/7/2015 - Darkened some Google Plus contact popup items. Added selection for dark or light sort/hangouts icon in the contacts header.
11/6/2015 - Fix for hangouts contacts and unread text color.
11/5/2015 - Initial Release.