style(ui): simplify secondary tools with grey styling

- Remove colorful emojis from Star/Donate text (⭐❤️)
- Simplify 'GitHub Tools' to just 'GitHub'
- Style secondary tools with subdued grey color (#6c757d)
- Smaller font size (0.9rem) to de-emphasize vs main navigation
- Grey hover effects instead of brand colors
- Main nav (Labs/Kits/PDF) remains prominent, tools are understated

book/assets/styles/style.scss

@@ -746,6 +746,29 @@ figcaption.quarto-float-caption-top {
         margin-right: 0.5rem;
         width: 1.2rem;
       }
+      
+      // Style secondary tools (star, heart, github) with subdued grey
+      &[href*="github.com"],
+      &[href*="opencollective.com"] {
+        color: #6c757d !important;
+        font-size: 0.9rem !important;
+        
+        &:hover {
+          color: #495057 !important;
+          background-color: rgba(108, 117, 125, 0.1) !important;
+        }
+      }
+    }
+    
+    // Style the GitHub dropdown toggle specifically  
+    .dropdown-toggle {
+      color: #6c757d !important;
+      font-size: 0.9rem !important;
+      
+      &:hover {
+        color: #495057 !important;
+        background-color: rgba(108, 117, 125, 0.1) !important;
+      }
     }
     
     // Style dropdowns in hamburger menu

book/config/_quarto-html.yml

@@ -79,13 +79,13 @@ website:
         href: https://mlsysbook.ai/pdf
         target: _blank
       - icon: star
-        text: "⭐ Star on GitHub"
+        text: "Star on GitHub"
         href: https://github.com/harvard-edge/cs249r_book
       - icon: heart
-        text: "❤️ Donate & Support"
+        text: "Donate & Support"
         href: https://opencollective.com/mlsysbook
       - icon: github
-        text: "GitHub Tools"
+        text: "GitHub"
         menu:
           - text: "✏️ Edit this page"
             href: https://github.com/harvard-edge/cs249r_book