Detect wells colors from CSS variables
Instead of storing the wells colors in JS (https://framagit.org/bnjbvr/kresus/blob/master/client/helpers.js#L73), that are shared by the wells and the charts, we could probably store them in CSS variables and access it with JS.
That would allow to easily modify kresus' skin by only modifying the CSS.
Although still experimental, CSS variables seem quite supported by all browsers now : https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables#Browser_compatibility.