you can use 2 ways
1 way – use inline style by clicking properties and enter your code in style box.
2 way – add custom class and use it on custom.CSS file in the path theme\style\css\custom\
An easy way to do this is to add
color: red
in the Style option box in the text box properties
Have you read any documentation about styling your app?
This should get you going. https://world.mendix.com/display/howto6/Create+a+custom+theme+with+the+Mendix+UI+Framework