Have you set it to use that theme in the product settings policy? Then do an agent wakeup and you should be able to see the theme update via the mfeepe.log.
Yes, that is how I have tried to set the theme. When I do the wakeup there is nothing in the log relating to the theme which is slightly confusing to say the least... I have tried setting it back to default, wake up, then custom and wake up but to no avail.
You should be able to see another theme in the log as themes are downloaded to clients. (Realised this when i used a Starfleet Academy logo as a test theme then discovered ALL themes are downloaded onto ALL clients!!!)
If not then its not applying.
if so then try this:
Copy the png from the actual theme on your ePO server- copy the zip file from the theme directory c:\Program Files (x86)\McAfee\ePolicy Orchestrator\DB\Software\Current\EETHEME\DAT\0000\themeID.zip, extract it and you want the background.png file.
Open it in MS Paint and edit it then save. (Re: How to change Theme Images? ) then Create your new theme: Create a new theme based on an existing theme - use McAfee Default and just upload the image you edited as the new background image.
Change that in product settings policy and apply to client.