Improve Color Settings in Accordion Block

[dcooney]

The current color settings (font and background) settings do not work in a practical website context. In most cases, users will require more granular control over the block styling otherwise they will be required to write custom CSS to complete the accordion styling. Element styling such as border/button colors, inline link styles and color contrast are problematic in the current implementation.

Proposal
Remove the font color setting and create a library of theme styles. Filterable theme styles would provide predefined color palettes and unique design aesthetics for the individual accordion items allowing for more control over the rendered output.

Implementation of the theme styling could work like the core Separator block does, where the available separator styles are radio selections in the block sidebar.

Accordion theming could potentially be extended at the theme level using hooks/filters.