Added hint to Attach a Fallback Value to a CSS Variable (https://guide.freecodecamp.org/certifications/responsive-web-design/basic-css/attach-a-fallback-value-to-a-css-variable/)
Added hint to Attach a Fallback Value to a CSS Variable (https://guide.freecodecamp.org/certifications/responsive-web-design/basic-css/attach-a-fallback-value-to-a-css-variable/)