See also
# nice explanation about the entire startx workflow
http://unix.stackexchange.com/questions/243195/what-desktop-environment-does-startx-run-and-how-can-i-change-it
# explanation about sessions
http://askubuntu.com/questions/62833/how-do-i-change-the-default-session-for-when-using-auto-logins

# list available desktop environments
ls -l /usr/share/xsessions
# show current login/display manager
cat /etc/X11/default-display-manager
# see also lightdm-greeter from Alternatives Configurator:
ls -l /usr/share/xgreeters
# how to restore Unity login greeter
cat /etc/lightdm/lightdm.conf
[SeatDefaults]
autologin-user=
allow-guest=false
greeter-session=unity-greeter -> add this line

# check the available session managers with
update-alternatives --list x-session-manager
# or get a more verbose description indicating which one is default with
update-alternatives --display x-session-manager
# shows the link to the default session manager
ls -l /etc/alternatives/x-session-manager
# change the default session manager by running
update-alternatives --config x-session-manager

# list available window managers
update-alternatives --list x-window-manager
# shows the link to the default window manager
ls -l /etc/alternatives/x-window-manager

# http://askubuntu.com/questions/62833/how-do-i-change-the-default-session-for-when-using-auto-logins
# list of available session types
ls -l /usr/share/xsessions
# see also ~/.dmrc for the current default selected session type
cat ~/.dmrc
# see also user defaults with
cat /var/lib/AccountsService/users/$USER

q: what to put in .xsession (e.g. for xrdp)?
a: pick from update-alternatives --list x-session-manager
warn: some of them won't work (something related to 3D graphics)
worked for me: xfce4-session, lxsession, mate-session, startlxde, openbox-session

# Zorin OS theme
# http://www.noobslab.com/2015/09/do-you-like-windows-10-look-but-love.html
# https://launchpad.net/~noobslab/+archive/ubuntu/themes?field.series_filter=xenial
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install windos-10-themes

# 9 Great XFCE Themes
# Ambiance theme for XFCE (with xfwm4)
Current XFCE theme:
grep -nr ThemeName .config/xfce4
When Settings -> Appearance doesn't open try running it from command line:
xfce4-appearance-settings

# change xfce desktop icon background/shadow
# see /usr/share/doc/xfdesktop4/README
# my ~/.gtkrc-2.0.mine
style "xfdesktop-icon-view" {
    XfdesktopIconView::label-alpha = 1

    fg[NORMAL] = "#ffffff"
    fg[SELECTED] = "#ffffff"
    fg[ACTIVE] = "#ffff00"
}
widget_class "*XfdesktopIconView*" style "xfdesktop-icon-view"