Does this work better for you?
type: custom:mushroom-template-card
icon: mdi
primary: LS50
secondary: '{{ ''On'' if is_state(entity, ''on'') else ''Off'' }} '
entity: switch.ls50
fill_container: false
multiline_secondary: false
layout: vertical
card_mod:
style: |
mushroom-card {
background: var(--card-background-color) url( '/local/ls50.png' ) no-repeat center 0px;
background-size: 42px 42px;
{% if is_state(config.entity, 'on') %}
{% else %}
filter: grayscale(100%);
{% endif %}
}