fix: update dark mode logo and add subtle gradient background

- Replace dark mode logo with new YourCompany SVG logo (white text)
- Update docs.yml to use separate logos for dark and light modes
- Add custom CSS with subtle linear gradient background for dark mode

Co-Authored-By: Kapil Gowru <[email protected]>

Author: devin-ai-integration[bot]

fern/docs.yml

@@ -90,9 +90,11 @@ theme:
   footer-nav: minimal
 
 logo:
-  dark: docs/assets/logo.svg
+  dark: docs/assets/logo-dark.svg
   light: docs/assets/logo.svg
   height: 20
   href: https://buildwithfern.com
 
 favicon: docs/assets/favicon.svg
+
+css: styles.css