There are different options to change the color theme and text fonts of the app.
Changing theme colors
You can customize the theme and accent colors of your app in the UX tab of the Editor.
Next to 'Color theme' you'll see a dropdown list of colors. Single colors set the theme of the app to that color only, and double colors change both the theme color and the accent colors.
Go to the Editor>UX>Branding tab.
Changing fonts and font sizes
Using the UX>Options tab, you can choose from a set of available fonts and also modify the font size.