--theme dark
https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Command-Line-Arguments-and-Settings

normally it should select dark mode automatically if your browser or systems preference

if it doesn't then probably there's some configuration that's saying to use light mode, try to find that setting and change it

or use the option that missionfloyd mentioned

you can also use the it by using the URL directly specify light or dark mode
http://127.0.0.1:7860/?__theme=dark
http://127.0.0.1:7860/?__theme=light

• if you don't like the look of the default theme you can set a custom theme Setting > User interface > Gradio theme with different color schemes
• if that is not enpugh there's extensions
• and if that is still not enough, there's always the option of user.css to have almost infinite customer ability
• if you want to go overkill then you can even combine it with JavaScript

if you reach this point you're probably an extension developer