error: an enum switch case label must be the unqualified name of an enumeration constant

As per Java docs

The Identifier in a EnumConstant may be used in a name to refer to the enum constant.

so we need to use the name only in case of an enum.

Change to this

switch (Prefs.getCardStyle()) {
    case COMPACT: rCompact.setChecked(true); break;
    case FLAT: rFlat.setChecked(true); break;
    case MATERIAL: default: rMaterial.setChecked(true); break;
}