For doing this first you need to go to this url and create icon by following the creating steps -After creating icon then click on download button. -you will see zip file then extract that file then you will see something like this enter image description here -then copy / move this file to [Your Project > Android > app > src > main > res > {Paste here}
for adding accent color / background color to notification there is an option in onesignal > push notification message section enter image description here enter image description here add in accent what you want on background.
-Add that image name which you have pasted to android folder (eg-enter image description here)
If you find any difficulties then please message me.