I have android and an iPhone. Both are adequately bright. Bold white and blue font on a grey & black background. Is it only on the My Day screen? Could you post a screenshot showing your issue?
Assuming you already have version 4.37.1, you might try uninstalling, restarting your iPhone, then installing again. If still no joy, report the issue to Garmin Support.
Same problem Apple 12 mini. Delete and Reinstall fixed it first try. Removing the icon from your Home Screen does not necessarily mean you deleted the app.
Notice how the bottom icons are bright but the MY DAY screen is darkened. All other screens are bright 
Much better than the manual 
