Hi Khai
You can add custom styling by creating your own stylesheets with classes and add them to your styling.
Probably you need to create a new class for the button and and your styling to this class.
I advice to follow this learning path:
https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass
and this documentation:
https://docs.mendix.com/howto/front-end/customize-styling-new/
For a quick fix you can use the styling area in the button properties and add your css there, probably you need something like padding; margin; or gap.
Good Luck!