I agree with all the sentiments that this needs to be addressed.
I think the best way to address this is not via a txt file but by a number of profiles that can be configured by the end user. These profiles would be configured in the online planner and synchronised to the app for navigation. The end user can then select the most appropriate profile for their current situation.
So for example, I might configure a profile for use on my bike where I use a ruggedised phone for navigation. In my car I have a built-in Android Auto/Carplay device which would use a different profile. For my camper I have a different set of requirements and therefore a different profile. The key is that I as the end user can define, easily save and then select the best profile for my scenario.
Settings would include, but certainly not limited to: map colours, road colours, contrast, route colour, line width and opacity, map scaling, zoom level at which POI's appear (by category), etc etc.
This way you can set a "sensible" default that can be used, probably by a majority of users, but also give "power" users the opportunity to customise their navigation experience.
We all use different devices in different environments, some are colour blind or have other visual impairments and others have very strong preferences. The development team will never be able to accommodate all of these scenarios so give the end user the option to choose.