How can I make rounded corners on grid?

I need to make the border of this grid rounded to fit my app theme. I'm trying with this css class but It doesn't works, what I need is a rounded border or corners on that grid and show them in the same color that the grid background.

.roundbox {
border-collapse: separate !important;
border: 10px #f4f4f4 solid !important;
-webkit-border-radius: 10px;
-moz-border-radius: 10px;
border-radius: 10px;

This is how appears in the app:

This is how is build:
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