Allow setting browser to dark theme in configuration

Clear and concise description of the problem

Every time I run browser tests, I manually click Toggle to dark mode when browser pops up.

It would be nice if I could set in configuration to always set it to dark theme for me.

Suggested solution

There could be configuration setting browser.theme that allows setting preferred theme.
Possible values could be 'light' or 'dark'.
Default value could remain 'light'.

    test: {
      browser: {
        theme: 'dark'
      },
    },

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

Start by tracing the browser-test configuration and the existing “Toggle to dark mode” control. Check how browser options are defined and validated, then run the relevant browser tests; done means a browser.theme setting accepts light or dark, defaults to light, and applies the selected theme.