[Skeleton] Increase wave contrast

Author: oliviertassinari

packages/material-ui-lab/src/Skeleton/Skeleton.js

@@ -49,7 +49,7 @@ export const styles = theme => ({
     overflow: 'hidden',
     '&::after': {
       animation: '$wave 1.6s linear 0.5s infinite',
-      background: 'linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.6), transparent)',
+      background: 'linear-gradient(90deg, transparent, rgba(255, 255, 255, 0.7), transparent)',
       content: '""',
       position: 'absolute',
       transform: 'translateX(-100%)', // Avoid flash during server-side hydration