Replace named colors by CSS variables

This replaces deprecated named colors by their matching CSS variable replacement.

Sidenote

Please note this leaves the custom named colors becasue replacing them is another story that I think should be discussed in a different MR, e.g. maybe we could have an API to access the palette colors from Vala in libadwaita to avoid copying them like what was done in the CSS. I have code ready for that (that copy the colors maualy in Vala) as well as some preliminary support for darker colors in dark mode, but I think this MR is self-sufficient.