Web page darkmode option in YAML

**Describe the problem you have/What new integration you would like**

An option in YAML to set web: to darkmode if not existed already...

**Please describe your use case for this integration and alternatives you've tried:**

Just a personal preference, like darkmode GUI better. Looked around, including the source code, but didn't see an option to set default darkrmode.

Or maybe there is a simple way to do it with `css_include:` or `js_include:`?

**Additional context**

maybe something like this

```
web_server:
port: 80
version: 1
darkmode: True
```