style: update ObjectivesBoard and SwimlanesBase for improved theming

- Replaced hardcoded colors with CSS variables in ObjectivesBoard for better theme consistency.
- Updated background color in SwimlanesBase to use new CSS variable for card columns.
- Enhanced button hover effects to align with the new theming approach.
- Minor adjustments to border colors for better visual coherence.

components/kanban/SwimlanesBase.tsx

@@ -264,7 +264,7 @@ export function SwimlanesBase({
               const isCollapsed = collapsedSwimlanes.has(swimlane.key);
               
               return (
-                <div key={swimlane.key} className="border border-[var(--border)]/50 rounded-lg bg-[var(--card)]/30">
+                <div key={swimlane.key} className="border border-[var(--border)]/50 rounded-lg bg-[var(--card-column)]">
                   {/* Header de la swimlane */}
                   <div className="flex items-center p-2 border-b border-[var(--border)]/50">
                     <button