Alerts not displaying properly (`--base-size-` vars not declared)

[roman910dev]

In version 5.5.0, github-markdown.css uses --base-size-8 and --base-size-16 vars for .markdown-alert and some other elements.

However, these are not declared so they defaut to 0, removing the padding from alerts.

github-markdown-light.css and github-markdown-dark.css don't use this vars, they use 8px and 16px instead, and alerts render properly.

[roman910dev]

For anybody bothered by this issue, the workaround is to add the following lines:

.markdown-body {
  --base-size-8: 8px;
  --base-size-16: 16px;
}

[aisevim]

#112 🤞

[rhysd]

This issue should be fixed by v5.6.0.