Fix color of attachement separator

Expected: (See https://www.figma.com/design/zftpgS6LjiczobJZ1GUNpt/Updates-to-Media---File-Upload?node-id=111-12505&m=dev)

<img width="729" height="401" alt="Image" src="https://github.com/user-attachments/assets/753addef-b289-4952-85cc-363558a619b4" />

Related: https://github.com/element-hq/element-x-android/pull/6519