allow custom text colors

[polyester]

This change would define five custom text colors, to match up with the five custom colors.

The CSS needs an "!important" in edit mode, to override another !important definition in bgcolor-blocks-layout.scss which says

[class^="block-editor-"] .block .text-slate-editor-inner span, body #page-add [class^="block-editor-"] .block .text-slate-editor-inner span {
  color: var(--theme-foreground-color) !important;
}

which would otherwise win, and render the description text always in theme-foreground-color.

Caveat: you have to make sure that the combinations of
highlight-custom-color-x and highlight-custom-text-color-x make up sensible and accessible combinations.

[polyester]

@danalvrz is this something that could go upstream in here, or should I keep it in our own theme?

[danalvrz]

@polyester We really should improve the color handling here, and fix the conflicts with VLT. Let me take a look at it this week 🙂