Fix: Make DropdownMenuSeparator background transparent based on theme (#9037)

This PR addresses issue #8993 by updating the `DropdownMenuSeparator` to
adjust its background color based on the current theme.

---------

Co-authored-by: guillim <guigloo@msn.com>
Co-authored-by: Lucas Bordeau <bordeau.lucas@gmail.com>

packages/twenty-front/src/modules/ui/layout/dropdown/components/DropdownMenuSeparator.tsx

@@ -1,9 +1,11 @@
 import styled from '@emotion/styled';
 
 const StyledDropdownMenuSeparator = styled.div`
-  background-color: ${({ theme }) => theme.border.color.light};
+  background-color: ${({ theme }) =>
+    theme.name === 'dark'
+      ? theme.background.transparent.light
+      : theme.border.color.light};
   min-height: 1px;
-
   width: 100%;
 `;