Add dark theme

[karlosmatos]

Adding mode toggle with updated UI colors to support dark theme.

[mlejva]

Hey @karlosmatos thank you for the PR. I checked it and there's a few more details that would need to get fixed before we can merge this PR:

The left up and bottom corners aren't rounded and it looks bad on the dark theme.

The code's text output isn't visible.

Borders of the selected input should be highlighted with some dark-friendly color

We could pick a better code's color theme for the dark theme.