feat: Add dark mode to the tutorials

[WiktorStarczewski]

Adds dark mode to the tutorials.
Mostly just enabled the feature in docusaurus, but also tweaked the inactive TOC text color (it was a little too dark for my liking), the section borders (too dark, not visible), the color of action buttons (black on grey looks bad), and width of the search button (it was cutting into the placeholder on my Brave/desktop vanilla Manjaro).

Also found a bug in Docusaurus where the search button dark theming was not preserved upon page reload. Had to resort to a hack to fix it, until fixed upstream. We can remove the hack if we don't like it, it's not a big deal, but light mode search in dark mode looks a little odd (I would still say acceptable probably but not perfect).

[Keinberger]

Dark mode has to be enabled on the miden-docs repository instead of individual source repositories.

How the docs are set up right now, is that the md files of each repository (listed here), are imported the miden-docs repository when running this workflow for deployment. That's how our Docs site is getting rendered, instantiated and getting deployed.

Therefore, dark mode has to be enabled on the miden-docs repository instead of the individual source repository. I already chatted with Ahmad, our designer, about designs and CSS specifications for Dark mode. It's on my to-do's, but so far just didn't get to it because of the post-Devconnect catch-up stress.