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
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