request to enlarge the accent marks or darken them so that they can be seen
I can use accent marks on my phone (an Android with Verizon service) if I can see them. But they are so small and so faint that I literally need to get out a magnifying glass to see them. If it is possible for you to do so, I would be grateful were you to enlarge and/or darken the accent marks so that they can be easily viewed. I appreciate your time and consideration.
Duolingo uses the underling OS for text rendering and I think there's no such option specifically to change accents' size (or their color). Actually, an accented character is an entity on its own (unicode provides some combining character but it's not how Duolingo implements accents).
I suppose they could implement a filter to change every accented letter with the normal character + the appropiate mark increased in size with a negative margin to make them combine, but it's not as easy as one could think.
They could add an option to increase the size of all the text in the app or you could increase it system-wide in the settings menu (on Android: Settings → Display → Font size or something like that)