I have the following alternative to @ahs solution. I implemented an input_boolean to control enable/disable the automations (if auto_darktheme is off, the theme will not change automatically)
# Set dark theme for the night
- alias: 'Set dark theme for the night'
initial_state: True
trigger:
- platform: sun
event: sunset
condition:
- condition: state
entity_id: input_boolean.auto_darktheme
state: 'on'
action:
- service: frontend.set_theme
data:
name: 'dark_theme'
# Set default theme for the day
- alias: 'Set default theme for the day'
initial_state: True
trigger:
- platform: sun
event: sunrise
condition:
- condition: state
entity_id: input_boolean.auto_darktheme
state: 'on'
action:
- service: frontend.set_theme
data:
name: 'default'