There are, surprisingly, several text editors for Windows Phone and Windows 10 Mobile - it seems as though making tweaks to plain text files is still a thing - think webmasters and bloggers. See below for details of a new UWP text editor that works brilliantly in a dark mode for AMOLED-screened phones.
Appy Text is a super fast + lightweight text editor. Immersive too - more content, less chrome. If you're a Notepad user but have been looking for a charmingly simple, modern and free alternative then look no further. Made (carefully) for PC, tablet and phone. And by going Premium for a small price, you get worthwhile extras. Like tabs. Auto-saving. Markdown support. Dark and sepia themes. There's a 30-day free Premium trial to help you decide.
And here's the app in action:
It's UWP, it's clean and fast, just what you want from a 2017 Windows text editor on a phone, eh?
Going 'Premium' gives you extra themes, as shown here - customise the editing experience according to your device and environment.
More examples of the cosmetic aspects you can consider - five overall themes and then font accents, sizes, etc.