You can edit the design of your page with CSS. If you are new to mendix, I would suggest to take a look on the Academy section. There are some helpful learning paths abut different topics.
For Mendix Versions 8 and above there is an easy to setup tool : https://docs.mendix.com/howto/front-end/calypso
if your question was targeted towards board styling possibilities, you should have a look at the already available classes in the atlas ui section https://docs.mendix.com/howto/front-end/atlas-ui (further reading on the bottom of the page :) )
good luck and have fun exploring!