explorer: initial UI re-themed to match Darkfi Book

This commit updates the Explorer application’s UI, providing a preliminary re-theming inspired by the Darkfi Book site. The updated design offers a familiar and visually appealing interface, along with improvements to responsiveness and usability on larger displays. These updates include changes to styling, layout, and modifications to key pages, such as the home page, block page, and transaction page.

This implementation represents the initial stage of improving the UI, making it more appealing for development and demos. It is not final, as further improvements will be made iteratively.