Update README.md

[pintaf]

As per my test, the default behaviour is to follow the browser dark mode

[ciminelli]

Hi @pintaf ! I'm not sure if the darkMode param gets set depending on the browser settings. Could you elaborate on your testing?

[pintaf]

Oh my god I feel dumb...
I had copy pasted the JS piece of code in README, which carries darkMode = true, but didn't noticed this
And since the docs said this is set to false by default, and it was in black for me, I thought it was taking the default browser setting.

So happy to update my PR in order to add this support of default darkmode being the browser mode, and could be overrided by darkmode setting