CSS Performance Optimizations for Grid

Vaadin's engineering team shares three CSS optimizations made for Grid and Tree Grid in Vaadin 25 to address severe rendering slowdowns with the new Aura theme. The key fixes were: replacing CSS box shadows (computed via custom properties) with regular borders, cutting rendering time by ~50%; switching from CSS part attribute selectors to class selectors, reducing selector match attempts by 3.5x and improving scrolling frame time from 90ms to 40ms; and registering CSS custom properties that use light-dark() with explicit <color> syntax via the Houdini @property API, reducing Aura rendering time by ~60%. Together these brought a 180-column grid's initial render from ~7.2s down to ~1s.