As of today to build the stylesheets we need to build N * M different files (where N in the colors and M the density mode) in every parts of Tuleap (tlp "framework", BurningParrot, plugins...) which have a non negligible impact on the build time and is an annoyance for developers due to the amount of boilerplate it requires.

Since Tuleap does not support IE anymore (request #18451), we can use CSS variables to achieve the same goal. Instead of building each time we need a stylesheet N*M files we can build once and only once N*M stylesheets containing the CSS variables and use only one stylesheet everywhere else.

This work can (and should) be done on both the colors and the density modes but dealing with the density modes first is easier due to the limited number of impacted variables and it already reduces the number of stylesheets we built by half (we have 2 density modes).