Add --theme=dark in startup options

[ostap667inbox]

Once the WebUI is launched, the light theme is launched by default.
Of course, there is nothing complicated to add manually in the address bar the parameter ?__theme=dark.
But it has to be done every time. The --theme=dark parameter which could be written in run.bat, as it is done, for example, in Automatic1111, is very much missing.

[lllyasviel]

added in 2.1.817