There are a few issues with this styles:

html.skin-theme-clientpref-night .metadata:not(.notheme) {
    background: inherit !important;
    color: inherit !important;
    border-color: var(--border-color-subtle, #c8ccd1) !important;
}

1. .metadata is a logical class that is used by many other tools.
2. Usage of !important rule which forces the usage !important in template styles to redefine it.
3. Too specific selector that can't be easily redefined.

So with this code, there are only two solutions for template editors, and both are bad:

• Make CSS rules like this one: https://ru.wikipedia.org/?diff=prev&oldid=137873932
• Remove .metadata class from templates at all.

Instead, there should not be such a rule. If there is an issue for some particular projects, and they really need it until templates and styles are fixed, they can add it to a local wiki's Common.css. Or it can be added to all projects by a bot, so local interface admins can remove it later.