Coko theme

I think we should merge the work I have so far on theming and UI, as there seems to be a ton of work involved here and the scope of this MR will become huge and (more) difficult to review.
Think of this MR as an improvement on what we currently have, rather than a finished thing. 😁

At least one person from YLD and one from Thin Slices should take a look at this, so that we're sure I didn't break anything for them.

Things to pay attention to:

• Removal of warning color (design#22 (closed))
• Removal of plain property on button.
• Changes in AppBar.