[nextra-theme-blog] Allow forcing a theme

[rgoomar]

I wasn't able to override the system theme with some script and figured this would be a good way to force it if you want light or dark. Currently, it defaults to system preference even with darkMode set to false or not configured.

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[rgoomar]

Tested this locally with modifying examples/blog/theme.config.js with:

forceTheme: 'light'

And running this and confirmed that even with system preference of dark, it defaults to light:

pnpm build:theme-blog
pnpm dev:theme-blog

[rgoomar]

@shuding Given that the darkMode flag only gives the toggle button (system theme is enabled by default), would you be open to renaming that to themeToggle and using a general theme config option that defaults to system but allows you to choose either light or dark explicitly?

[gigamesh]

+1 - would love to be able to set the default to dark (using docs theme in my case)