You can do this by exiting the server, selecting it in the Multiplayer list, then clicking "Edit" toward the bottom of the screen.
Ensure the box which says "Server Resource Packs" (underneath the IP box) is set to Enabled, not Disabled or Prompt.
Reconnect to the server! When you first connect with resources enabled, the server will automatically download the pack, which may take a few seconds.
Toggle this to make the website fluid width.