Add the ability to customize the color of alert boxes

I would love to have the ability to use custom colors for :::alert::: boxes, instead of just red, yellow, green, and blue? So instead of :::danger::: we could use rgb, hex, or even color names:  :::red:::, :::rgb(1,0,0):::, etc.

At least one other issue in the repo mentions this but the issue was closed.