add dark theme for nimbus.guide (#5564)

docs/the_nimbus_book/mkdocs.yml

@@ -16,9 +16,20 @@ theme:
     - navigation.top
     - content.tabs.link
   palette:
-    scheme: default
-    primary: orange
-    accent: amber
+    - scheme: default
+      primary: orange
+      accent: amber
+      toggle:
+        icon: material/brightness-7
+        name: Switch to dark mode
+    - scheme: slate
+      primary: black
+      accent: light blue
+      toggle:
+        icon: material/brightness-4
+        name: Switch to light mode
+extra_css:
+  - stylesheets/extra.css
 
 # Support urls previously used by mdbook
 use_directory_urls: false
@@ -29,7 +40,6 @@ docs_dir: src
 
 markdown_extensions:
   - admonition
-  - meta
   - pymdownx.details
   - pymdownx.highlight:
       anchor_linenums: true

docs/the_nimbus_book/src/stylesheets/extra.css

@@ -0,0 +1,6 @@
+[data-md-color-scheme="slate"] {
+    --md-hue: 180;
+    --md-default-bg-color: hsla(var(--md-hue), 0%, 0%, 1);
+    --md-footer-bg-color: hsla(var(--md-hue), 0%, 0%, 1);
+    --md-footer-bg-color--dark: hsla(var(--md-hue), 0%, 0%, 1);
+  }