Ever since list-picker's first usage in Tuleap, most of its CSS rules have been scattered in FlamingParrot and TLP. Originally, this was due to the lack of CSS Custom properties. We didn't want to have to write 12 different stylesheets (because of colors and condensed mode), so the style rules just piggybacked on top of FlamingParrot general styles and in TLP. Not only it makes it very hard to find the styling rules and maintain them, but it also means they were copy-pasted and slightly tweaked in each case.

We should put a stop to this and move all of the rules in list-picker's stylesheet. Nowadays, we are able to use CSS custom properties and most of the properties for colors are already exposed, so we no longer have this constraint and we can write the rules only once.