Design a color palette and use it consistently

[graue]

Most of the colors we use in BikeHopper have been selected pretty ad hoc.

We should come up with a palette of colors that we can use consistently, akin to the default color palette of Tailwind where it's easy to for example just plug in green-300 somewhere (and couple it with dark:green-800 for dark mode), and so on.

We could just use the Tailwind default palette but I do like the theme color of "BikeHopper green" for instance as seen in the background of the top bar.

I think this is a blocker on #76: it would be tedious to properly support dark mode without having something like this.