refs. https://github.com/TryGhost/Casper/pull/631
- refined colors of links and separator dots for various cases (light/dark mode/with and without header background image)
- fixed bookmark card hover style
- ran CSSComb
refs https://github.com/TryGhost/Ghost/issues/9311
- move `.kg-card-markdown` styles and add deprecation notice
- add support for new `.kg-post` wrapper
- add support for `.kg-image` and `.kg-image-wide/full` image variants
- add support for image captions