Well if you have edited the wplms theme files directly then your changes are gonna be lost for sure after updating theme in both case automatic and manually .
If you added your custom css in wp-admin -> appearance -> customize -> then you do not need to worry it wont lost on update as it is saved database.
We always recommends our users to make customizations using child themes and actions and filters provided in theme .
NOTE : customizations added in wplms customizer plugin also remain safe on updates .