If you are within the main Simpods theme, not a child theme, you can overwrite and modified the classes dependently, although it is not recommended as every time you update Simpods, your work will be removed.
If you are working with the Simpods child theme, in theory you can overwrite the some place-holder classes, but the core classes can’t be overwritten.
If you install Simpods in another child theme, you can overwrite and modified the classes dependently. Unfortunate. You will have to manually update Simpods but we are working on a remote update functionality to make it automatic.