I'm getting similar error and can't find a way to solve it. Also several updates of Home Assistant in the past didn't solve the error. It occurs when I try to use a Theme from HACS. The normal "dark mode" I can select without problems.
Logger: homeassistant.core
Source: components/frontend/init.py:468
First occurred: 22:05:26 (2 occurrences)
Last logged: 22:06:38Error executing service: <ServiceCall frontend.reload_themes (c:3a56d98e44a93e7a2d1b007bdf56e1fc)>
Error executing service: <ServiceCall frontend.reload_themes (c:9dead53fda6d714bb182dbde3c8dd34f)>
Traceback (most recent call last):
File "/usr/src/homeassistant/homeassistant/core.py", line 1648, in catch_exceptions
await coro_or_task
File "/usr/src/homeassistant/homeassistant/core.py", line 1667, in _execute_service
await cast(Callable[[ServiceCall], Awaitable[None]], handler.job.target)(
File "/usr/src/homeassistant/homeassistant/helpers/service.py", line 738, in admin_handler
await result
File "/usr/src/homeassistant/homeassistant/components/frontend/init.py", line 468, in reload_themes
new_themes = config[DOMAIN].get(CONF_THEMES, {})
KeyError: 'frontend'
Update 06.03.2022
My problem was solved here: Switching to HACS themes not possible - #3 by ryzn