Improve appearance

[dou-du]

• Choose final colours and style; understand the widget behaviour on small screens
• Understand if we want/it is possible easily to make the colours, size, ... customisable (in this case, maybe the colours should be set via a dictionary, per element, like on SSSP?)
• Allow to pass a dictionary from python to change the label of elements (e.g. on SSSP, adding subscripts etc).

[dou-du]

Why not use bokeh? see https://docs.bokeh.org/en/latest/docs/gallery/periodic.html
This will also make issue#4 easy.
(We have adapted the bokeh example for plotting and selecting element dependent values).
Maybe one can get somehow the javascript code and not depend on bokeh?

Are the periodic table plots from materbialscloud search app and the SSSP not packaged and reusable somewhere?