First of all, you can put it in a single layout grid as @Marcel Groeneweg said to reduce the space between layout grids. You can also control how that page will look by pressing the Design mode button in the upper right corner.
Coming to the other question, Yes with CSS you can do anything on the front-end as far as the Mendix allows. You can do this in two ways.
First, you can write CSS codes in the style section of the properties of the thing you want to apply (Container, Button, other widgets, etc.).
Second, you can write CSS codes on the CSS page in the Styling folder in App explorer. Don't forget to give the Class name of the thing you want to implement (Container, Button, other widgets, etc.) At the same time, to add the Module Styling folder, you can right-click on the styling folder and select "App and all modules".
I have attached screenshots below,
I hope it helps.
Put them in a single layout grid. Nested layout grids don't get that much padding. Or just use a single layout grid.