I wrote a plug-in for RoundCubeMail 1.7 that patches dark-mode to work in the message viewer and the message editor.
I hope someone can use it as well as myself.
rpc-scandinavia/rpc_rcm_patches: Packagist page (https://packagist.org/packages/rpc-scandinavia/rpc_rcm_patches)
The plugin looks good. One thing I don't see is a way to switch to light / default mode in case there is a message that needs to be view in the original form.