I activated the system monitor and I changed the sensor name in the git project example (i.e. sensor.processor_use instead of sensor_raspi_cpu) but the result is the same 
following the code
type: 'custom:button-card'
entity: sensor.raspi_temperature
icon: 'mdi:raspberry-pi'
aspect_ratio: 1/1
name: HassOS
styles:
card:
- background-color: '#000044'
- border-radius: 10%
- padding: 10%
- color: ivory
- font-size: 10px
- text-shadow: 0px 0px 5px black
- text-transform: capitalize
grid:
- grid-template-areas: '"i temp" "n n" "cpu cpu" "ram ram" "sd sd"'
- grid-template-columns: 1fr 1fr
- grid-template-rows: 1fr min-content min-content min-content min-content
name:
- font-weight: bold
- font-size: 13px
- color: white
- align-self: middle
- justify-self: start
- padding-bottom: 4px
img_cell:
- justify-content: start
- align-items: start
- margin: none
icon:
- color: |
[[[
if (entity.state < 60) return 'lime';
if (entity.state >= 60 && entity.state < 80) return 'orange';
else return 'red';
]]]
- width: 70%
- margin-top: '-10%'
custom_fields:
temp:
- align-self: start
- justify-self: end
cpu:
- padding-bottom: 2px
- align-self: middle
- justify-self: start
- '--text-color-sensor': '[[[ if (states[sensor.processor_use].state > 80) return "red"; ]]]'
ram:
- padding-bottom: 2px
- align-self: middle
- justify-self: start
- '--text-color-sensor': >-
[[[ if (states[sensor.memory_use_percent].state > 80) return "red";
]]]
sd:
- align-self: middle
- justify-self: start
- '--text-color-sensor': '[[[ if (states[sensor.disk_use_percent].state > 80) return "red"; ]]]'