I have an Nvidia Shield TV and use ES File Explorer as my file manager. Normally I would never be caught dead using this app but there aren't as many file manager apps for Android TV as there are for phones, nevertheless ones that are as comprehensive as ESFX.
In ESFX there are 2 ways that I know you can change your theme.
The first is to click the theme button. The app has however put it behind some kind of app download lock to where you don't get the feature unless you download one of their sponsored apps. I would have no problem doing this but unfortunately app linking to the Play Store isn't supported on Android TV and the interface is not optimized for Android TV so I cannot click on it even if I wanted to.
The second option is to download a theme on the Play Store. This is also not available.
Does anyone know how to get around this annoying issue or perhaps is there an app for Android TV that rivals ESFX in functionality?
The first thing you need to do is uninstall that bloatware junk off your Shield, then buy your Shield a drink to apologise for installing that shit on it.
Seriously ES is bloatware that should be removed from the play store.
Try X-Plore or Total Commander. I've been using X-Plore for ages, it was a great improvement even when ES wasn't hostile junk.