FWIW Firaxis is very unlikely to use a pure black background with an inside color due to confusion risk with independent powers. They had the same visual language with CIV 6 (city states had a black bg with colored middle based on yield type) so realistically they would probably avoid it. Plus the IPs in vanilla 7 have the middle coloring with the black BG too, so that would be too confusing to introduce a civ with the same pallette.
In that case you could use the yield type to add the middle color/faction color to the units without getting rid of the black. This would color the glow, give you a future proof color scheme, and it would also alleviate a lot of the confusion of units. Separating units by yield type colors allows you to see which units are from which city-state at a glance, so you dont have to hover over them.
Kinda like this. Maybe leaving the city name text white (Its still consistent because it would be a mechanic only used for independent powers, black and white labels). It wouldn't just fix the glow problem, it would also address the lack of an intuitive way to discern what city state troops are from