Hi,
There an easy one to fix I guess, not sure if it has been reported yet, I searched a bit but it seemed not, my apologies if it has already.
When giving a custom color for an event, dhtmlx add css inline property background to .dhx_title, .dhx_body and .dhx_footer.
The issue with doing that it override the .dhx_footer background property with the image.
Would suggest to only set background-color instead of background.
Regards