I'm developing an Onboard theme and colour scheme which makes use of alpha transparency, with the intent of applying it to the pantheon-greeter (based on LightDM). In the process, I've discovered that LightDM does not appear to support alpha transparency.
In order to test this issue, please open a terminal and execute the following:
sudo -i
xhost +SI:localuser:lightdm
su lightdm -s /bin/bash
onscreen-settings
and then proceed to set a high transparency setting for Onscreen's window background (or better yet, tick the 'no background' option under /window/transparency) and reboot your machine. In the ubuntu/pantheon-greeter (both use Onboard), summon the on-screen keyboard from accessibility menu (in the top right hand corner) and behold, opaqueness!
Regards,
Lee Hyde.
P.S.: I am reliably informed that the underlying XEmbed window may itself pose a blocker for implementing transparent themes in LightDM greeters (or in any other circumstance were Onboard is executed in XEmbed mode). I'm told that the XEmbed window background will likely not inherit any alpha transparency and will this show through (it's a light grey a'la you common law garden gtk+ window). I believe that this is a separate issue (as I saw no signs of this light grey background) and will file a big against Onboard (or whom soever it concerns) in due course.
Unfortunately LightDM can't help here - it's just run a standard X server and the greeter inside it. So to support transparent windows either X needs to be informed of this or perhaps this is the function of a window manager (which pantheon greeter will be acting as in this case). Sorry I don't know enough about transparent windows to say where exactly.