Allow easy color-based rebranding of the UI

Currently, the block explorer is available solely in a grey-based implementation. It would be nice to have the UI color scheme configurable, for example in the env file or config file so that anyone who wants to implement it can configure the colors as needed.

Colors of interest could be the boxes background (dark grey currently), text color, empty background color (light grey/white), as well as options for the dark/light modes (when implemented via other task #334 (closed))