I've just added a dark theme to Sigil -- see thumbnail.
If anyone else wants to try doing this then do the following:
* Go to Github with this link:
QDarkStylesheet
* Copy all the css code on the GitHub page.
* Using any text editor like Notepad or TextMate create a new file on your desktop called "qt_styles.qss" and paste all the copied code into that new file and save it.
* Open Sigil and go to
Plugins > Manage Plugins > Open Preferences Location and copy your new "qt_styles.qss" file into that directory.
* Restart Sigil.
Other dark themes you can try that work on Sigil:
DarkOrange