I would be happy to add in a windows dark mode specific Qt .qss stylesheet to the CodeView editor to increase contrast but since I do not have access to a Windows box and can not see the issue or compare against other non Qt apps in dark mode, I have no way to test anything.
Sigil can already read and use a .qss stylesheet, so if someone with access to Windows can generate a piece of stylesheet, I would be happy to include it.
See:
https://bugreports.qt.io/browse/QTCREATORBUG-15865
and
https://forum.qt.io/topic/104646/why...is-transparent
for ideas and approaches.