refactor: update button colors in CallToAction and Hero components for improved contrast

- Changed link button color from cyan to white in agents, home, and network CallToAction components
- Updated network Hero secondary button from dark to gray outline variant

src/pages/network/Hero.tsx

@@ -100,7 +100,7 @@ export function Hero() {
                 as="a"
                 to="https://threefold.info/mycelium_network/docs/"
                 variant="outline"
-                color="dark"
+                color="gray"
                 target="_blank"
                 rel="noopener noreferrer"
               >