Let admins specify colors in LCH inside the configuration

[LukasKalbertodt]

Currently we only allow sRGB hex strings. But since #796 we convert them to LCH internally anyway. So why not allow specifying colors in LCH in the config file, e.g. primary = "lch(30% 0 20)" or sth like that. This is likely not really all that important for most. Most CIs only use sRGB colors. But it might still be useful if the CI color is brighter than we allow and admins then want to reduce its brightness without reducing its chroma.