Select language and light/dark mode for signin, signup, and error page

[Sjoerd82]

Select language and light/dark mode for signin, signup, and error page

[casbin-bot]

@tangyang9464 @JalinWang @imp2002

[Sjoerd82]

Use tag "enhancement".

[hsluoyz]

@Sjoerd82 can you make a PR?

[Sjoerd82]

@Sjoerd82 can you make a PR?

I'm neither a Go nor React developer, and have no prior knowledge of the source code.
That didn't stop me to have a look though. And it seems that this enhancement can probably best be realized in web/src/auth/EntryPage.js.

.. it was at this point that I realized setting language is already possible! (ex. ...&language=de)
The only fix is to document it :-)

To make setting the theme possible too, something like this:

  getThemeParam(params) {
    // "/page?theme=dark"
    const theme = params.get("theme ");
    if (theme !== null) {
      setTheme(theme, ????);
...

Not sure where best to call it, and what initThemeAlgorithm does.

Probably best if an actual Casdoor dev implements this...