refactor: Declare default CSS symbol colors under :host as well

This is apparently needed when the docs are served under a shadow DOM (for example, Backstage seems to do this, and `:root` variables aren't picked up).

See https://developer.mozilla.org/en-US/docs/Web/CSS/:host.

PR-186: #186

src/mkdocstrings_handlers/python/templates/material/style.css

@@ -31,7 +31,7 @@
 }
 
 /* Symbols in Navigation and ToC. */
-:root,
+:root, :host,
 [data-md-color-scheme="default"] {
   --doc-symbol-attribute-fg-color: #953800;
   --doc-symbol-function-fg-color: #8250df;