refactor: CSS adjustments [Accordion]

src/QUI/Bricks/Controls/Accordion.css

@@ -7,14 +7,17 @@
 
 .quiqqer-accordion-item {
     border-bottom: 1px solid #ddd;
-    margin-bottom: 1rem;
 }
 
 .quiqqer-accordion-item-header {
     align-items: flex-start;
     cursor: pointer;
     display: flex;
-    font-size: 1.25rem;
+    margin-top: 0;
+    margin-bottom: 0;
+    padding-top: var(--spacing, 1rem);
+    padding-bottom: var(--spacing, 1rem);
+    font-size: var(--accordion-header-font-size, 1.5rem);
 }
 
 .quiqqer-accordion-item-header-icon {