Explorer: Add a dark mode (compatible with Beam Wallet colors) (#1994)

* Add loading and title animations

Useless but fun animations...

* Define all colors as CSS variables

Will help creating dark/light modes later on.

* Add folder for DApp version

* Revert "Merge branch 'master' into Explorer"

This reverts commit 6fa6487, reversing
changes made to 15f5bf7.

* Add Dark mode

Add dark theme colors and a toggle button. Also adjust some CSS and remove some of the previous tentative title animations.

* Adjust dark background to Beam Wallet colors

Use same background gradient as Beam Wallet (and other small adjustments).