GTK - Enable/set dark theme on a per-application basis

With gtk+ ≥ 3.12 you can load a specific theme and its variant (dark, light) on a per-application¹ basis via the environment variable GTK_THEME=theme:variant. As per the gtk+ reference manual:

GTK_THEME.  If set, makes GTK+ use the named theme instead of the theme that is
specified by the gtk-theme-name setting [...] It is also possible to specify a
theme variant to load, by appending the variant name with a colon, like this:
GTK_THEME=Adwaita:dark.

So, to load² the dark variant you would run:

GTK_THEME=Adwaita:dark gedit

Likewise, to achieve the opposite (when the default theme is dark), you load the light variant:

GTK_THEME=Adwaita:light gedit

Note that if you want to use it via a custom launcher (.desktop file) you'll have to prepend env to the command in the Exec line:

Exec=env GTK_THEME=Adwaita:dark eog %U

^{1: Worth noting that - as per the devs decision - newer gnome-terminal has its own configuration via menu > preferences and it ignores the theme. Also, since this is rather new stuff, some gtk+ 3 applications might not (yet) honor the GTK_THEME environment variable.
2: This doesn't seem to work if you already have a running instance of that application e.g. if nautilus is already running in dark mode then running GTK_THEME=Adwaita:light nautilus will open a new nautilus window but still in dark mode. I don't know if this is a feature or a bug...}

For GTK+-3 applications, you can enforce the dark theme variant using GtkSettings' settings.ini:

$ mkdir -p $HOME/.config/gnome-terminal/gtk-3.0 # the path before gtk-3.0 is arbitrary
$ cat >> $HOME/.config/gnome-terminal/gtk-3.0/settings.ini << EOF
[Settings]
gtk-application-prefer-dark-theme=true
EOF
$ echo 'alias gnome-terminal="XDG_CONFIG_HOME=$HOME/.config/gnome-terminal gnome-terminal" >> $HOME/.bashrc

For non-GTK+ applications like vnc, you can still enforce dark window decorations by setting the _GTK_THEME_VARIANT X property of type UTF8_STRING to dark. To do this with xprop, type the following command and click the window afterwards:

$ xprop -f _GTK_THEME_VARIANT 8u -set _GTK_THEME_VARIANT "dark"

xprop can also select windows by the WM_NAME property (the title bar label) or by window id:

$ xprop -f _GTK_THEME_VARIANT 8u -set _GTK_THEME_VARIANT "dark" -name "Spotify Premium - Linux Preview"
$ xprop -f _GTK_THEME_VARIANT 8u -set _GTK_THEME_VARIANT "dark" -id 0x380002b

To get window ids and names, use xlsclients -l.

Here is a bash script I have used to launch a application with a different theme. Haven't used it in years though, so I don't know if it will work with the current GTK.

#!/bin/bash
# lauch a gtk application with a different theme
# set GTKRCFILE variable to your favourite theme
GTKRCFILE=Clearlooks
GTK2_RC_FILES=/usr/share/themes/"$GTKRCFILE"/gtk-2.0/gtkrc "$@"

Here is a bit of info I got off the ubuntu fourms archives. Not sure if it will be a problem. (here) There is also a blog post detailing this method a little more. (here) and (here). There is also a similar question on this site that has already been answered. (here)

Under gnome, apps get their theme from the gnome-settings-daemon. This instantaneously applies any theme change to all active applications, making per-appplication theme changing impossible.