Although Windows 10 includes an option to switch to the light and dark modes within the Personalization settings, if you want to use the light colors globally, while the Mail app with the dark color scheme, the app allows to switch between modes manually within its settings.
How to enable dark mode in the Mail app
To use the dark color scheme with the Mail and Calendar apps on Windows 10, use these steps:
Click the Settings (gear) button in the bottom-left.
Click the Personalization option in the top-right.
Under the “Colors” section, select the Dark mode option.Quick Tip: If you select the Use my Windows mode, the Mail app will switch automatically to the dark theme when enabling the Dark option on Settings > Personalization > Colors.
Once you’ve completed the steps, the Mail and Calendar apps will switch to the dark color scheme.