Added automatic dark mode to the widget

[SebastianGode]

A lot of pages nowadays respect the CSS prefers-color-scheme attribute to change the colors of the website automatically based on the users browser or system settings.
As the widget didn't have any CSS logic to respect that and looked quite ugly with the dark mode I added some CSS to make the widget also look good if the user requests the dark mode of a page.
I also adjusted the existing background a bit so that we don't have white-on-white for a normal page.

This is how it will look in dark mode (prefers-color-scheme: dark):

And this is how it will look in light mode (prefers-color-scheme: light):

Note that the links are blue as well if not visited before.

If you have any suggestions feel free to write comments.

[tischrei]

looks much better

[realaravinth]

Looks good!

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Hello @SebastianGode,

I apologize for the delay, I just got back from an unavoidable personal leave. I will get to your PRs and issues over the next couple of days.