When a module group is created in a template and has styling applied via the Edit CSS option, the CSS will still appear even after that module group is set to become a global group within that template. However, if that same global group is added to a different template, the styling will not carry over; you will need to re-add the styling as you did in the original template because the CSS lives on the template, not on the global group itself.
For example, go to Content > Design Manager > Find a template to edit
- Group some modules together by clicking Group at the top > Click on existing modules to include in the group > Click Create Group
- Next, click on the gear for that group and select Edit CSS > Enter your desired CSS to apply to the module group > Click Done
- Click on the gear once again for the same module group and select Make Group Global > Name and Save the new group
- Now that your global group hase been created, click on the gear one last time on the newly created global group and select Edit CSS > Here you will see the originally entered CSS code from before making the module group global.
But, if you add this global group to a new/existing template, that original CSS will not transfer over; you will need to add fresh CSS code to the global group on the next template as the CSS lives on the template, not on the global group.