You can switch Newton between light and dark, choose which of its 16 languages it's shown in, and make text bigger. On iPhone and iPad, Newton has no text size setting of its own: it uses the text size you set for your device.
Steps
Desktop
Open Settings: press ⌘, on Mac, or Ctrl+, on Windows and Linux.
Go to the General tab.
Under Appearance, choose a Theme: System, Light or Dark.
Next to Language, choose System or a language.
To make everything bigger or smaller, use the View menu: Zoom In or Zoom Out. Reset Zoom (⌘⇧0 on Mac, or Ctrl+Shift+0 on Windows and Linux) goes back to the normal size. On Windows, press Alt to show the menu bar.
iPhone & iPad
Open the menu and tap the Settings gear at the bottom.
Tap Appearance, then under Theme choose System, Light or Dark.
Go back, tap General, then Language. Choose System default or a language.
For bigger or smaller text, change your device's text size: in the Settings app, go to Accessibility › Display & Text Size › Larger Text. To change it for Newton only, add Text Size to Control Center, open it while Newton is on screen, and choose Newton Only.
Android
Open the menu and tap the Settings gear at the bottom.
Tap Appearance, then under Theme choose System, Light or Dark.
Go back, tap General, then Language. Choose System default or a language.
For bigger or smaller text, change the font size in your phone's settings. Newton follows it.
Good to know
The 16 languages are English (US), English (UK), Dansk, Deutsch, Español, Français, Italiano, 日本語, 한국어, Norsk bokmål, Português (Brasil), Português (Portugal), Русский, Svenska, 简体中文 and 繁體中文.
System follows your device's language, or its light or dark setting.
Changing the language doesn't change your emails. To read an email written in another language, translate it.
On desktop, the language also applies to the menu bar and the tray menu.