docs: change primary color from blue to indigo (#444)

* fix: change primary color from blue to indigo

Align with the cpp-linter organization hub site branding.
Indigo (#4051b5) better matches the cpp-linter logo and identity.

* chore: remove redundant header background-color rule

Now that the palette primary color is indigo, the manual CSS override
for .md-header and mobile nav drawer is no longer needed.

Author: shenxianpeng

docs/stylesheets/extra.css

@@ -2,11 +2,6 @@ th {
     background-color: var(--md-default-fg-color--lightest);
 }
 
-.md-header,
-.md-nav--primary .md-nav__title[for="__drawer"] {
-    background-color: #4051b5;
-}
-
 @keyframes heart {
 
     0%,

mkdocs.yml

@@ -34,7 +34,7 @@ theme:
   palette:
     # Palette toggle for automatic mode
     - media: "(prefers-color-scheme)"
-      primary: blue
+      primary: indigo
       accent: cyan
       toggle:
         icon: material/brightness-auto
@@ -43,7 +43,7 @@ theme:
     # Palette toggle for light mode
     - media: "(prefers-color-scheme: light)"
       scheme: default
-      primary: blue
+      primary: indigo
       accent: cyan
       toggle:
         icon: material/lightbulb-outline
@@ -52,7 +52,7 @@ theme:
     # Palette toggle for dark mode
     - media: "(prefers-color-scheme: dark)"
       scheme: slate
-      primary: blue
+      primary: indigo
       accent: cyan
       toggle:
         icon: material/lightbulb