Fix variables when used in an embedded window like on backstage

docs/css/material.css

@@ -4,7 +4,7 @@
 }
 
 /* Custom admonition: preview */
-:root {
+:root, :host {
   --md-admonition-icon--preview: url('data:image/svg+xml;charset=utf-8,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M15.5 12a3.5 3.5 0 1 1-7 0 3.5 3.5 0 0 1 7 0Z"/><path d="M12 3.5c3.432 0 6.124 1.534 8.054 3.241 1.926 1.703 3.132 3.61 3.616 4.46a1.6 1.6 0 0 1 0 1.598c-.484.85-1.69 2.757-3.616 4.461-1.929 1.706-4.622 3.24-8.054 3.24-3.432 0-6.124-1.534-8.054-3.24C2.02 15.558.814 13.65.33 12.8a1.6 1.6 0 0 1 0-1.598c.484-.85 1.69-2.757 3.616-4.462C5.875 5.034 8.568 3.5 12 3.5ZM1.633 11.945a.115.115 0 0 0-.017.055c.001.02.006.039.017.056.441.774 1.551 2.527 3.307 4.08C6.691 17.685 9.045 19 12 19c2.955 0 5.31-1.315 7.06-2.864 1.756-1.553 2.866-3.306 3.307-4.08a.111.111 0 0 0 .017-.056.111.111 0 0 0-.017-.056c-.441-.773-1.551-2.527-3.307-4.08C17.309 6.315 14.955 5 12 5 9.045 5 6.69 6.314 4.94 7.865c-1.756 1.552-2.866 3.306-3.307 4.08Z"/></svg>');
 }
 

docs/usage/customization.md

@@ -127,7 +127,7 @@ if it supports light and dark themes,
 otherwise just override the variables at root level:
 
 ```css title="docs/css/mkdocstrings.css"
-:root {
+:root, :host {
   --doc-symbol-parameter-fg-color: #df50af;
   --doc-symbol-attribute-fg-color: #0079ff;
   --doc-symbol-function-fg-color: #00dfa2;

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;