[docs] Adapt primary color to match uMap one

docs/stylesheets/extra.css

@@ -0,0 +1,3 @@
+:root {
+  --md-primary-fg-color: #323e56;
+}

mkdocs.yml

@@ -22,17 +22,20 @@ theme:
   palette:
     - scheme: 'default'
       media: '(prefers-color-scheme: light)'
+      primary: 'custom'
       toggle:
         icon: 'material/lightbulb'
-        name: "Switch to dark mode"
+        name: 'Switch to dark mode'
     - scheme: 'slate'
       media: '(prefers-color-scheme: dark)'
-      primary: 'blue'
+      primary: 'custom'
       toggle:
         icon: 'material/lightbulb-outline'
         name: 'Switch to light mode'
   features:
     - navigation.sections
+extra_css:
+  - stylesheets/extra.css
 markdown_extensions:
   - pymdownx.magiclink
   - admonition