I’ve opened bug some time ago, but unfortunately still got no response.
Maybe somebody also got same issue but found workaround and can share it here.
To do not duplicate problem description can be found here: Setting OVERWRITE_SETTING_theme-custom-css env variable doesn't take effect · Issue #29224 · RocketChat/Rocket.Chat · GitHub
In short: theme-custom-css cannot be set through env variables because of ‘-’ sign.
Thanks in advance for any halp.