Dark-mode needed in `https://haystack.deepset.ai/`

[SupRaKoshti]

I have been reading tutorial in https://haystack.deepset.ai/tutorials.
But it's really hard to read because the white background and lite font color.
So can we have Dark Mode options in all overall website so visitor or learner can read tutorial easily.

[harshdotcom]

@kacperlukawski Could you please assign this task to me? It is a high-priority task.

[kacperlukawski]

@harshdotcom I moved this issue to haystack-home, as it's not only related to tutorials. Just assigned you, feel free to tag me for a review later.