Add CSS variables theme for html formatter

The HTML formatter now supports using a CSS variables theme which will not output any colors and leaves it on you to define the colors using the CSS variables. Following is the list of variables you will have to fill out.

:root {
  --pre-bg-color
  --pre-fg-color
  --toggle-fg-color
  --braces-fg-color
  --brackets-fg-color
  --dt-number-fg-color
  --dt-bigint-fg-color
  --dt-boolean-fg-color
  --dt-string-fg-color
  --dt-null-fg-color
  --dt-undefined-fg-color
  --prototype-label-fg-color
  --dt-symbol-fg-color
  --dt-regex-fg-color
  --dt-date-fg-color
  --dt-buffer-fg-color
  --function-label-fg-color
  --array-label-fg-color
  --object-label-fg-color
  --map-label-fg-color
  --set-label-fg-color
  --object-key-fg-color
  --object-key-prefix-fg-color
  --class-label-fg-color
  --collpase-label-fg-color
  --getter-label-fg-color
  --circular-label-fg-color
  --weakset-label-fg-color
  --weakref-label-fg-color
  --weakmap-label-fg-color
  --observable-label-fg-color
  --promise-label-fg-color
  --generator-label-fg-color
  --blob-label-fg-color
  --unknown-label-fg-color
}

Features

• add cssVariables theme (8ce3166)

Full Changelog: v0.4.2...v0.5.0