Add darkmode again

[lucacasonato]

We should add darkmode again like we had on the v1 site. It should be based on prefers-color-scheme again.

We have to update:

• main layout (material-ui theme)
• code blocks (so cant use github-gist theme anymore)
• charts (once/if Moved to new chart library and readded normalized charts #39 is merged the apexcharts theme)
• invert deno logos

[ry]

Sure - but let's please keep the CSS as minimal as possible.

[lucacasonato]

Fixed by #60.