Build a webpage, host it on the device.

Doesn't sound like a self hosting thing really. More of an app that plays local videos that you can also skin, thing.

Some DIY options:

• You could build an App with Flutter that allows you to play videos from the filesystem.
• Create a PWA (web app), that gets served from an external Cloud source or serve it from a local Raspberry Pi.
• You may even be able to just dump a single app.html file in a folder. It could contain your HTML, CSS, and JS. If your iconography is simple you could just embed them as SVGs, otherwise reference images the same as the videos. You may run into file permission errors going this route. For example I tried running an html file I created on my Android device, and Chrome complained but another app called HTML viewer didn't. Probably just had to drop the file in a directory that the Chrome App had access to but I didn't want to put anymore time into it.