I understand you are trying to enlarge the font size in the UI of the native app, is that correct?
The font size setting in the Editor setting is only applied for Test Scripts, Pre-request Scripts, and Response Pretty View, but not for the whole user interface.
I found a similar issue having been discussed here - You can enlarge the font by the shortcut as shown below (please check Settings > Shortcuts for your OS):
I don’t think there is any setting in Postman to control the font colour.
Having said that, you might be able to control the colour contrast etc by monitor setting, e.g.