[Urgency color] Prepare special color palette for dark mode

[kocio-pl]

Urgency color makes tasks hardly readable under dark mode (it's basically unusable) and violates the concept of low-light eyes-friendly theme, so we need a new palette for this:

[nekohayo]

Yup, somewhat known issue, not just for the urgency plugin but also the "color-code the background based on tag's color" setting in the built-in preferences... See also #608 for the big picture and links to other related issues.