Hi Christina, I believe the striped and bordered datagrids are already supported in the default mendix themes. You need to add the following classes to your datagrid datagrid-striped and datagrid-bordered.
You can checout the mendix UI showcase to see for yourself how this datagrid looks like, see: https://ux.mendixcloud.com/theme-creator.html and go to design elements section and then buttons datagrids.
If you want to know to create the srtipes and borders yourself, you can check out the _datagrids.scss file in the theme\styles\sass\lib\components folder in your mendix project.
If you want to make it default you need to change the _datagrids.scss file in the theme\styles\sass\lib\components folder in your mendix project.
Checkout the Datagrid Striped section and copy or replace the elements to the Datagrid Default section.