@fairmarketvalue,
When testing this I actually stumbled across an old thread on this same subject that I responded to 
Unfortunately, in this theme I stated that his would be fixed after the NTP we were working on [at that time] and that clearly has not aged well, as it looks like the issue remains. Unless there is some specific configuration I’m unaware of, the NTP setting in Brave seems to override anything that the theme/extension is attempting to display on the NTP. I think this is best exemplified in this example, where I went to Appearance --> New Tab Page --> New Tab Page Shows and selected Blank page in Brave and applied the Damnation Creek Trail theme, which resulted in this:
Where you can clearly see the image appear in the border of the window/tabs but not on the actual page. I also tried going to NTP → Settings → Show background images and toggling this option “off”, which resulted in this:
I will ask the team about this as, in my opinion, there should be an additional option in one of these places where you can select Theme and display the image(s) that the current theme has set for the NTP.