feat: adding dark mode functionality to blog

[AsherThomasBabu]

This PR is intended to add dark mode functionality to the blog and closes #56 on merge

In order to override the third party theme, I had to bring the index.html and header.html files to the local repo. I'm not sure if this is a right practice, but seems like the only way.

I also had to remove the SVG background on the hero of the blog, this could either be added in as CSS or left out as such.
The Illustration on the hero also seems a little shabby, If a higher quality image can be linked, that too can be added.

Looking for further input on what else can be improved.

This is a snip of how the current dark mode looks like.

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AsherThomasBabu

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[tachyons]

Any update on upstreaming the changes ?