💡 Idea: Allow direct referencing of theme colours in new styles?

[Keilin Olsen]

It'd be super helpful to define custom styles for a diagram (or class) which properly adapt to theme changes, eg. by referencing the colour palette categories or indices

Hypothetical syntax (a few options - but entirely off the cuff to demonstrate intent):

example1: "allowing theme indices as valid CSS names" {
  style: {
    fill: "base"                     // use the base colour, select the index based on depth as usual
    stroke: "supportingA"  // use the analogous accent for stroke, also selecting index based on depth 
  }
}

example2: "(preferred but more effort) using a theme keyword for access" {
  style: {
    fill: theme.base
    stroke: theme.supportingA
  }
}

example3: "specifying an index directly" {
  style: {
    fill: theme.base.1
  }

  note: |md
    (Not sure the best way to encode indices here - using dot form seems weird for numbers, but don't think array syntax is supported by d2?)
  |
}