Where can custom user styles be created or obtained?

[Andoriax]

I'd love to tweak the display theme styles.
Any other themes / "styles" available?

[itkach]

User-defined style is a regular text file containing CSS, can be created in any text editor. Here, for example, is the dark theme built-in into dictionaries created by mw2slob: night.css. mw2slob includes it into dictionaries it creates, but this could be a user style. Give it a try - copy it, change some colors, copy to your device and see how it looks. aard2-web can be used to open articles in a desktop browser with dev tools to inspect articles HTML and experiment with CSS.