Proposal: migrate colors to oklch/CSS 5

[michaelfester]

Is your feature request related to a problem? Please describe. Handling variations of colors can be tricky (e.g. hover colors) and currently requires defining new CSS variables, instead of applying transforms (e.g. lightness) to a base set of colors. CSS 5 color transformations can simplify this, and make accessibility easier to control

Describe the solution you'd like Move variables to okhcl, and use CSS 5 colors transforms for variations. As a result, remove unnecessary variables.