It looks like the container on the left is using the ‘Card Action’ building block. This building block has pre-populated classes which styles the icons.
In Studio Pro analyse the properties of all the elements (study the class property) and then compare to see what is missing.
The styling of a Mendix app is based on Bootstrap. You can find the CSS/SASS files in the theme folder of your project in the local folder.
Some info on out of the box classes: https://docs.mendix.com/howto/front-end/styles
Do CSS magic yourself: https://docs.mendix.com/howto/front-end/calypso